Updates PigToThrift to support more Pig map (String) key conversions

This is helpful when target Thrift map key type is something other than STRING or ENUM.

Conflicts:
	pig/src/test/java/com/twitter/elephantbird/pig/util/TestPigToThrift.java
Fixes merge bug
Fixes mvn test invocation
Simplifies code at expense of pushing keyField type test into loop
Makes use of Field.isBuffer()
Merge pull request #339 from sagemintblue/hazen/pigtothrift_map_key_conversions

Updates PigToThrift to support more Pig map (String) key conversions