have some helper methods to build easily gems with java extensions

[mkristian]

lot of projects use rake-compiler to build gems with extension. to compile the java extension code using the declares jars from the gemspec file it needs some method exposing the classpath for the compiler.

there are two cases

• the gem just needs the runtime jars for compiling
• the gem needs the runtime jars and jruby itself

something like

Jars.classpath
Jars.compile_classpath
Jars.test_classpath

this avoid using ruby-maven directly in
guyboertje/jrjackson#35
sparklemotion/nokogiri#1253
https://github.com/mkristian/psych/blob/jruby/Rakefile#L54

[mkristian]

this should be done by now.