[refactoring] Complete switch to Scala language

Plugin is now completely written in Scala 2.11. All plugin
configuration is made using the Scala's Stackable Trait Pattern,
separating each configuration concern into a different trait.

Implements the generation of proto descriptors in a new task,
'generateProtoDescriptors'.

ScalabuffExtension now allows the user to fail fast the build in case
protoc compiler is not found on path.

Remove redundant task clearProto.
[tooling] Change gradle wrapper

Make gradle-wrapper.properties point to a location that downloads
binaries, sources and javadoc, allowing IDE's to show sources in sync
with class files.
[tooling] Add gradle wrapper native scripts

Add scripts generated from the execution of the wrapper task.
[feature] Add example projects

Add single and multi project examples to show the usage of the
plugin from the perspective of a client.