Add plugin descriptor to main source set (#3)

See https://github.com/etiennestuder/gradle-plugindev-plugin/pull/9
Extract constant MAIN_GENERATED_RESOURCES_LOCATION
Set bintray user and key if defined through project properties
Document bintray user and key default project properties
Merge pull request #12 from etiennestuder/pr/9

Make plugin descriptor part of source set output, look up default properties for bintray credentials.