#32: Empty message support

Remove checks for zero fields in proto compiler and runtime schema generator.
Add integration tests for no-field message serialization & deserialization.
Merge branch 'master' into fix/message-without-fields

Conflicts:
	protostuff-runtime/src/main/java/io/protostuff/runtime/RuntimeSchema.java
#32: Reformat test
Merge pull request #65 from kshchepanovskyi/fix/message-without-fields

#32: Empty message support
Merge pull request #68 from kshchepanovskyi/fix/enum-runtime-tags

#25: @Tag support for protostuff-runtime
#25: @Tag support for protostuff-runtime - fix enum value deserialization