added tests for jruby-complete.jar

* travis just pick jdk7 to run those tests
* use uri:classloader://META-INF/jruby.home as jruby-home
* the antlib/extra.xml will be generated while generating the test/pom.xml from test/pom.rb
  and uses jruby19.index as source of test files (```cd test; rmvn validate```)
* the test-helper needed some improvements to accommodate jruby-complete
* jrubyc test is skipped since JRuby::Compiler.compile_argv only works with filebase jruby installations

Sponsored by Lookout Inc.
when testing File.file? the impl needs to use the FileResource.file?

the old implementation used FileResource.stat.file? which throws a
'File or Directory not found' and does no testing. the new implementation
follows the pattern used for File.directory? and File.exists?

Sponsored by Lookout Inc.