Add shading support

[wxiang7]

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.

[eed3si9n]

Awesome! Thanks for this contribution!

[eed3si9n]

Could we move this into package sbtassembly? If Pants is using that name, I don't want to collide with it.

[wxiang7]

I'm afraid we can't, since process method in JarProcessor is package private.
I'm not sure why they use such a method signature.

[eed3si9n]

Do you mind adding some documentation on README?

[eed3si9n]

I feel like ShadeRule should a sealed trait, and Rename, Remove, KeepOnly should each be a case class under object ShadeRule. I can do that refactoring.

[wxiang7]

Will add more doc after all these refactoring :)

[wxiang7]

hi @eed3si9n, could you please help review the refactored code when you're convenient. thanks :)

[eed3si9n]

Refactoring looks good!
Do you mind adding a scripted test? Here's explanation of scripted - http://eed3si9n.com/testing-sbt-plugins

[wxiang7]

hi @eed3si9n , scripted tests added.

[eed3si9n]

Thanks again.