Support for building "-SNAPSHOT" artifacts

[mockitoguy]

It is now possible to conveniently build and locally install snapshot artifacts using:

./gradlew snapshot

See all details in the design spec.

Tested with Mockito project:

~/mockito/src$ ./gradlew snapshot
Parallel execution is an incubating feature.

> Configure project : 
  Building version '2.18.1-SNAPSHOT' (value loaded from 'version.properties' file).

> Task :compileJava 
Note: Some input files use or override a deprecated API.
Note: Recompile with -Xlint:deprecation for details.
Note: Some input files use unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

BUILD SUCCESSFUL in 12s
56 actionable tasks: 51 executed, 5 up-to-date

[epeee]

Thank you! merging...