Add toString method
Use MessageStringGenerator
Fix test for toString
Reformat generated java code
Use const references for arguments of client functions
Write unit test for gen_argument_type
Merge pull request #449 from unnonouno/feature/client-const-reference

Feature/client const reference: fix #444
Merge pull request #434 from unnonouno/feature/java-to-string

Add toString method to user deinfed classes