Fixed instructions

Tried the installation on Mac OSX 10.9.4 according to your instructions when two problems occurred:

**I needed to set the compiler-paths when configuring `llvm-3.3` explicitly:**
```Shell
./configure --enable-optimized --prefix=/Users/michaelriedel/Developer/llvm-3.3.src/build/
...
configure: error: Selected compiler could not find or parse C++ standard library headers.  Rerun with CC=c-compiler CXX=c++-compiler ./configure ...
```
**I needed to use `sudo` due some permission issues:**

```Shell
LLVM_CONFIG_PATH=../llvm-3.3.src/Release+Asserts/bin/llvm-config python setup.py install
...
error: could not delete '/Library/Python/2.7/site-packages/llpython/__init__.py': Permission denied
```