Merge ldb into LLVM trunk.

This should properly merge lbd into the LLVM trunk. Everything is the same except that the book under `docs/BackendTutorial` instead of in the root directory. I have also fixed `make gh-pages` to work with the new setup.

This pull is HUGE since it adds the entire LLVM development history and merges the lbd repository into the LLVM history. I did a little bit of git magic to perfectly merge the lbd development history even though it is now in a subdirectory (`docs/BackendTutorial`). This means that git will still *cleanly fast-forward* when merging this pull request.

You should also be able to easily update to the latest LLVM trunk version by adding any LLVM git mirror (such as <git://github.com/llvm-mirror/llvm.git>) as a remote and directly merging from it:

```
$ git remote add llvmtrunk git://github.com/llvm-mirror/llvm.git
$ git fetch
$ git merge llvmtrunk/master
```

(You only have to do `git remote add ...` the first time)

Let me know if you have any other questions.

We should now be ready to start adding code on top of LLVM!

I also modified LLVM's README.txt to indicate the purpose of this LLVM fork.