BLD: Add 2to3 caching, move the build cache logic into use_build_cache.py
I tested this script all the way back to 0.7.0. The build cache code was a lot of noise
to shove into setup.py, I'm glad to pull it back out.

a "goto_ver.sh" script is now really easy to do, and very useful for benchmarking
and bisecting regressions.

.bashrc:

``` bash
# Use the pandas build cache
export BUILD_CACHE_DIR="/home/user1/tmp/.pandas_build_cache/"
if [ ! -e $BUILD_CACHE_DIR ]; then
    mkdir $BUILD_CACHE_DIR ;
fi

function cpdev {
scripts/use_build_cache.py # rewire setup.py with build_cache
python ./setup.py clean;
python ./setup.py develop; 
git checkout setup.py # restore setup.py
}
```
