Correct references to the `com.github.jruby-gradle` group for dependencies

I've also made the implicit dependency on the base plugin noted in the
documentation, see #10 for more details
Merge pull request #11 from rtyler/readme-fixup

Correct references to the `com.github.jruby-gradle` group
Bump the version for the next bit of development
Add a logback configuration to avoid spitting so much garbage to the term when testing
Explicitly set the jruby-complete up for the compile configuration

Fixes #12
Merge pull request #15 from rtyler/issues/12-jruby-dep

Explicitly set the JRuby compile-time dependency
Add caveat for #14
Update changelog with 0.1.2
Bump the jar plugin dependency up to 0.1.8+

Fixes #18
Bump the plugin version for the next release
Mkae the shadow plugin an explicit runtime dependency for the jar plugin

Shadow is required to make executable jars which seems to me to be a pretty key
part of the plugin

Fixes #17
Note that shadow is now an explicit dependency of the plugin
Rolled back compatibility supprot to JDK6