Improved error messages for debugging.
Address GHC warning.

GHC 7.8 warns that join shouldn't be shadowed when importing Prelude, so
we explicitly hide it. This generates its own warning... so that's neat.
Define emptyMsgContext.
More tweaking to quotation mark escaping.
Unit tests for message code generation.

Test parsing, MD5 computation, and Haskell code generation for messages
defined in the actionlib_msgs package, and a message including string
constants (whose quotation escaping behavior is easy to get wrong!).

Fixes #21