Add support for gemfile style :github (or :git) dependencies

[swistaczek]

rmvn package
[INFO] Scanning for projects...
/Users/ernest.bursa/Projects/meta-store/.tesla.Mavenfile
NoMethodError: undefined method `github' for #<Java::OrgApacheMavenModel::Dependency:0x7cac93fe>
      do_dependency at /Users/ernest.bursa/.rvm/gems/jruby-1.7.17@global/gems/maven-tools-1.0.7/lib/maven/tools/dsl.rb:1067
               each at org/jruby/RubyHash.java:1341
      do_dependency at /Users/ernest.bursa/.rvm/gems/jruby-1.7.17@global/gems/maven-tools-1.0.7/lib/maven/tools/dsl.rb:1066
             do_gem at /Users/ernest.bursa/.rvm/gems/jruby-1.7.17@global/gems/maven-tools-1.0.7/lib/maven/tools/dsl.rb:1203
                gem at /Users/ernest.bursa/.rvm/gems/jruby-1.7.17@global/gems/maven-tools-1.0.7/lib/maven/tools/dsl.rb:1161