Upgrade to sbt 0.13.1.

Upgrade zinc project itself to sbt 0.13.1. The only breaking change was
removal of sbt.Util.counted method in 0.13. I just copied those few lines
of code from the old version of sbt directly to zinc.
Bump sbt version zinc depends on to 0.13.2-SNAPSHOT

This is the version that zinc uses to build its own components. It's
not the version used for the project itself.
Add support for nameHashing and recompileOnMacroDef options.

Add support for two new incremental compiler options introduced in
sbt 0.13.2:

  * `nameHashing` which enables new, experimental incremental compilation
    algorithm
  * `recompileOnMacroDef` which controls heuristic related to how
    macro definitions and their dependencies are handled

Note that since `recompileOnMacroDef` is turned on by default we introduced
`recompileOnMacroDefDisabled` command line option which changes the
default value to be off.
Merge pull request #42 from gkossakowski/name-hashing

Add support for `nameHashing` and `recompileOnMacroDef`