Introduce IncrementalCompilerOptions.

To be described and split into smaller chunks.
Log API diffs using ShowAPI and java-diff-utils library.

Implement displaying API changes by using textual representation
of an API (ShowAPI) and good, old textual diff algorithm. We are
using java-diff-utils library that is distributed under Apache 2.0
license.
Merge branch 'inc-compiler-options' into api-diff
Do not minimize APIs if api debugging is enabled.
Display API if `apiDebug` option is enabled.
Add a test for showing API for Java circular structures.
Use `Map<String, String>` for inc options in API.

Specific changes are:
  * Remove IncrementalCompilerOptions interface; based on mailing
    list feedback we decided that static interface is not the best
    way to go because it's hard to evolve it by adding new options
    we use `java.util.Map<String, String>` instead
  * Rename `IncrementalCompilerOptions` to `IncOptions` that
    reads a lot better
  * Since passing `java.util.Map<String, String>` not very convenient
    we convert it immediately to `sbt.inc.IncOptions`
Merge branch 'inc-compiler-options' into api-diff

Conflicts:
	compile/inc/src/main/scala/sbt/inc/Compile.scala
	compile/inc/src/main/scala/sbt/inc/Incremental.scala
	compile/integration/src/main/scala/sbt/compiler/AggressiveCompile.scala
	compile/integration/src/main/scala/sbt/compiler/IncrementalCompiler.scala
	interface/src/main/java/xsbti/compile/Setup.java
	main/actions/src/main/scala/sbt/Compiler.scala
	main/src/main/scala/sbt/Defaults.scala
	main/src/main/scala/sbt/Keys.scala
Use sbt 0.12.2.
Let Maven poms to be published.

It's kind of amazing that those were not published for more than 10
months and IDE team didn't notice...
Include `inc` in sbt version.

We do that to make sure that projects like IDE are built
with the right version.