Add error message for checking postgres install configuration
Add error message for checking postgres install configuration

MADlib needs to be installed on a postgres with python configured. If
postgres is not configured with the `--with-python` option, madpack
installation will failed for not finding $libdir/plpython2 directory.
This commit add some instructions when this failure happens.

Closes #186

njayaram2/incubator-madlib,madlib/madlib,njayaram2/madlib,orhankislal/incubator-madlib,njayaram2/incubator-madlib,iyerr3/incubator-madlib,apache/incubator-madlib,njayaram2/madlib,hpandeycodeit/incubator-madlib,orhankislal/madlib,orhankislal/madlib,orhankislal/incubator-madlib,orhankislal/incubator-madlib,apache/incubator-madlib,hpandeycodeit/incubator-madlib,iyerr3/incubator-madlib,njayaram2/incubator-madlib,hpandeycodeit/incubator-madlib,apache/incubator-madlib,njayaram2/madlib,madlib/madlib,iyerr3/incubator-madlib,orhankislal/incubator-madlib,apache/incubator-madlib,orhankislal/incubator-madlib,orhankislal/madlib,madlib/madlib,njayaram2/incubator-madlib,hpandeycodeit/incubator-madlib,hpandeycodeit/incubator-madlib,njayaram2/madlib,njayaram2/madlib,orhankislal/madlib,iyerr3/incubator-madlib,orhankislal/madlib,njayaram2/incubator-madlib,madlib/madlib,apache/incubator-madlib,iyerr3/incubator-madlib,madlib/madlib