Added page with LLVM 3.4-based (experimental) install instructions

On the old website, the Experimental Guide with instructions on how to install KLEE with LLVM 3.4 was outdated and hard to find, so I created a new one.
I also made a few updates and corrections to the Getting Started page and the Readme.

Two remarks:
* In the new Getting Started guide, the instructions to install klee-uclibc have been changed: instead of using the git repo, a tar file should be downloaded. Is this the preferred way now? In the LLVM 3.4 guide, I used the git repo method.
* In the new LLVM 3.4 guide, in the package installation section, I simply added libboost-all-dev. I didn't check which subpackages are really required (only libboost-program-options-dev and libboost-system-dev?), but if someone knows it, it would be better to list only those.