Use pip for installing numpy scipy matplotlib

brew install numpy fails on Mac OSX Mountain Lion. 

pip install is the favoured way of installing the libraries. The 
caveat is to ensure that the python executable being used is the 
one installed via homebrew and not the default one present on Macs. For that, we run the exec $SHELL command.

The brew install pip is redundant as the "brew install python" 
command already supplies pip with it.