Start removing dependence on gflags.

Reviewed in https://codereview.appspot.com/7628044/.

This only removes the dependency from the core library, a second CL will
update all the samples.
diff --git a/oauth2client/util.py b/oauth2client/util.py
index ee6a100..90dff15 100644
--- a/oauth2client/util.py
+++ b/oauth2client/util.py
@@ -22,9 +22,11 @@
 ]
 __all__ = [
   'positional',
+  'POSITIONAL_WARNING',
+  'POSITIONAL_EXCEPTION',
+  'POSITIONAL_IGNORE',
 ]
 
-import gflags
 import inspect
 import logging
 import types
@@ -38,12 +40,13 @@
 
 logger = logging.getLogger(__name__)
 
-FLAGS = gflags.FLAGS
+POSITIONAL_WARNING = 'WARNING'
+POSITIONAL_EXCEPTION = 'EXCEPTION'
+POSITIONAL_IGNORE = 'IGNORE'
+POSITIONAL_SET = frozenset([POSITIONAL_WARNING, POSITIONAL_EXCEPTION,
+                            POSITIONAL_IGNORE])
 
-gflags.DEFINE_enum('positional_parameters_enforcement', 'WARNING',
-    ['EXCEPTION', 'WARNING', 'IGNORE'],
-    'The action when an oauth2client.util.positional declaration is violated.')
-
+positional_parameters_enforcement = POSITIONAL_WARNING
 
 def positional(max_positional_args):
   """A decorator to declare that only the first N arguments my be positional.
@@ -93,10 +96,11 @@
         def my_method(cls, pos1, kwonly1=None):
           ...
 
-  The positional decorator behavior is controlled by the
-  --positional_parameters_enforcement flag. The flag may be set to 'EXCEPTION',
-  'WARNING' or 'IGNORE' to raise an exception, log a warning, or do nothing,
-  respectively, if a declaration is violated.
+  The positional decorator behavior is controlled by
+  util.positional_parameters_enforcement, which may be set to
+  POSITIONAL_EXCEPTION, POSITIONAL_WARNING or POSITIONAL_IGNORE to raise an
+  exception, log a warning, or do nothing, respectively, if a declaration is
+  violated.
 
   Args:
     max_positional_arguments: Maximum number of positional arguments. All
@@ -107,9 +111,9 @@
     being used as positional parameters.
 
   Raises:
-    TypeError if a key-word only argument is provided as a positional parameter,
-    but only if the --positional_parameters_enforcement flag is set to
-    'EXCEPTION'.
+    TypeError if a key-word only argument is provided as a positional
+    parameter, but only if util.positional_parameters_enforcement is set to
+    POSITIONAL_EXCEPTION.
   """
   def positional_decorator(wrapped):
     def positional_wrapper(*args, **kwargs):
@@ -119,9 +123,9 @@
           plural_s = 's'
         message = '%s() takes at most %d positional argument%s (%d given)' % (
             wrapped.__name__, max_positional_args, plural_s, len(args))
-        if FLAGS.positional_parameters_enforcement == 'EXCEPTION':
+        if positional_parameters_enforcement == POSITIONAL_EXCEPTION:
           raise TypeError(message)
-        elif FLAGS.positional_parameters_enforcement == 'WARNING':
+        elif positional_parameters_enforcement == POSITIONAL_WARNING:
           logger.warning(message)
         else: # IGNORE
           pass