Add a JVM method signature builder

[kud1ing]

In order to resolve Java methods, one needs to declare type signatures like "(Ljava/lang/Object;)V" which is a bit unwieldy.

It would be nice to have an easy way to generate those strings.
See https://docs.oracle.com/javase/1.5.0/docs/guide/jni/spec/types.html

[treyzania]

Partially done in #30.