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:
  * Add a method to `xsbti.compile.Setup` that returns incremental
    compiler options as a `java.util.Map<String, String>`. We considered
    statis interface instead of a `Map` but based on mailing
    list feedback we decided that it's not the best way to go because
    static interface is hard to evolve it by adding new options.
  * Since passing `java.util.Map<String, String>` not very convenient
    we convert it immediately to `sbt.inc.IncOptions`
  * Add options argument to various methods/classes that implement
    incremental compilation so in the end options reach
    `sbt.inc.IncOptions` object
  * Add `incOptions` task that allows users to configure incremental
    compiler options in their build files. Default implementation of
    that tasks returns just `IncOptions.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`.