Introduce incremental compiler options.

Introduce a way to configure incremental compiler itself instead
of underlying Java/Scala compiler.

Specific list of changes in this commit:
  * Introduce a new API interface `IncrementalCompilerOptions`
    that describes currently available options.
  * Add a method to `xsbti.compile.Setup` that returns
    `IncrementalCompilerOptions`
  * Add options argument to various methods/classes that implement
    incremental compilation so in the end options reach `inc.Incremental`
    object
  * Add `incOptions` task that allows users to configure incremental
    compiler options in their build files. Default implementation of
    that tasks returns just `IncrementalCompilerOptions.DEFAULT`
  * Both system property `xsbt.inc.debug` and `IncOptions.relationsDebug`
    trigger debugging of relations now. In the near future, we should
    deprecate use of `xsbt.inc.debug`.
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`