API-Update gradle-1.6

Using ConfigurationContainers and TaskContainers 'create()' instead of 'add()'.
Enable use of external binding declarations.

Use external binding declarations from 'src/main/xjb/*.xjb' (by convention).
Configure alternative binding directory by
  generateSchemaSource.bindingDir 'src/main/alternative-xjb'
Merge pull request #17 from stephanheinze/master

Avoid warnings with gradle-1.6