TurboJPEG: Thread-safe error message retrieval

Introduce a new C API function (tjGetErrorStr2()) that can be used to
retrieve compression/decompression/transform error messages in a
thread-safe (i.e. instance-specific) manner.  Retrieving error messages
from global functions is still thread-unsafe.

Addresses a concern expressed in #151.
diff --git a/doc/html/functions_vars.html b/doc/html/functions_vars.html
index 8373eac..7d45f87 100644
--- a/doc/html/functions_vars.html
+++ b/doc/html/functions_vars.html
@@ -24,7 +24,7 @@
  <tr style="height: 56px;">
   <td style="padding-left: 0.5em;">
    <div id="projectname">TurboJPEG
-   &#160;<span id="projectnumber">1.5</span>
+   &#160;<span id="projectnumber">1.6</span>
    </div>
   </td>
  </tr>