[FEATURE] allow external call to R instead of rpy2

git-svn-id: https://msproteomicstools.googlecode.com/svn/trunk@263 db641661-14cc-1e01-c2f8-e77ac6f8a696
[FEATURE] add option to use external R call on the CLI

git-svn-id: https://msproteomicstools.googlecode.com/svn/trunk@264 db641661-14cc-1e01-c2f8-e77ac6f8a696