First cut at including zinc build in the uber-build.

* Add new operations `sbt-nightly`
* Add hooks to not build the ide itself.
* Instead of forking a git repo for zinc, just copy the local files.
* Use zinc's default scala versions.properties in lieu of the one from scala if doing a nightly.

Quick review from @skyluc and continued tweaking after this.
Fixes from @skyluc's comments
* Use checkAvailability correctly
* Do quick sanity check on zinc local build directory before copying.
* Always copy the build scripts from local zinc directory.
Now load sbt version from the .properties file.

* Eclipse will use the sbt version found in our properties file.
* The default in the config is used if we don't rebuild sbt.

Review by @skyluc
Add the ability to swap in/out the sbt properties file used in dbuild.

* Can swap the properties file used in dbuild
* Allows you to build IDE against different sbt version numbers
* Can use alternative branches of sbt/sbinary/sbt-republish from the default.
* Example nightly build for sbt 0.13.x (-SNAPSHOT).