A few improvements to java_gen from the plane...

Reviewer: @rlane @yotamhc

Here's another pull request with a set of improvements I put together yesterday on the plane. It includes
 * unit tests for the discriminator parser feature + and struct/enum params
 * a check-java and a package-java target for the make file
 * various documentation enhancements
 * blacklisting of various useless types from the input (of_uint32)
 * and an experimental change to upcast the java representation of values to the next bigger type (e.g., byte -> short) to preserve the correct unsigned values. This representation has a few trade-offs of its own (e.g., it's not obvious from the type that a 'short' value that's really a uint8 will only accept values 0-255.

I won't be checking emails often during next week. Feel free to merge the changes if they look ok, or, Yotam if you'd like make changes and push a PR with the changes (or otherwise I'll do it next week).

Have a good time
-A.