Use ThaumCraft API repo as submodule

In light of issue #265 it was determined that RailCraft included outdated ThaumCraft API classes, leading to crashes whenever someone tried opening the Thaumonomicon. The two obvious solutions are:

* Do not include ThaumCraft API classes (hard)
* Update the included ThaumCraft API classes (simple)

I have chosen the second solution, by adding the API repository as a submodule, as is the case for a number of other included projects.

This pull request includes a commit to remove all existing API classes, and a second commit that adds the submodule in place of the old API package.

After merging, don't forget to do a `git submodule update` (and maybe even `git submodule init`).

Tested on a local forge installation with `Thaumcraft-1.7.10-4.2.1.4.jar`, `buildcraft-6.0.18.jar` and the `Railcraft_1.7.10-9.3.3.3-unofficial.jar` local build.