Add shading support, #156.
In this PR, a new assembly setting key is introduced, namely assemblyShadingRules
e.g.

assemblyShadingRules in assembly := Seq(
  Shader.rename("com.google.protobuf.**" -> "com.google.protobuf3.@1")
    .applyToCompiling
    .applyTo("com.google.protobuf", "protobuf-java", "3.0.0-alpha-3.1")
)

assemblyShadingRules consists of a sequence of shading rules. And there're three types of shading rules in total, they're: Shader.rename, Shader.remove, Shader.keepOnly. For each shading rule, it can be applied to a list of targets: applyToCompiling means applying it to current module, and applyTo means applying it to some dependency artifact.

These three types of shading rules originated from jarjar rules, for details please go to this link.