Adding connection classes to main __init__.py

Now that connection classes can be passed as arguments on Target
instatiation, they are part of the user-facing API and are marked as
such by exporting them in the main __init__.py
diff --git a/devlib/__init__.py b/devlib/__init__.py
index 4ebe082..fa64b89 100644
--- a/devlib/__init__.py
+++ b/devlib/__init__.py
@@ -16,3 +16,7 @@
 from devlib.instrument.netstats import NetstatsInstrument
 
 from devlib.trace.ftrace import FtraceCollector
+
+from devlib.host import LocalConnection
+from devlib.utils.android import AdbConnection
+from devlib.utils.ssh import SshConnection