Update README for new tap location

Github repository ownership has changed from edc to rdkit.
Remove version that is redundant with version scanned from URL

Based on warning from `brew audit`: Stable: version 2013.09.2 is redundant with version scanned from URL
Switch to tar.gz download instead of zip

Based on warning from `brew audit`: Use GitHub tarballs rather than zipballs
Clarify swig dependency as build only
Use preferred head url definition style
Use build instead of ARGV to check options
Add option to build postgresql cartridge
Build static libs as required by postgresql cartridge
Merge pull request #6 from mcs07/master

Tidy up formula and add postgresql option