Update Mac install script

* Always brew install python.
* Explicitly set virtualenv executable.
* Drop pip installation; installing python via brew has us covered.
* Warn the user of potential connectivity problems when UPNP fails.
* Add autoenv support for mac
* Always activate our virtual environment
* Only on mac, tell users to run openbazaar with --disable-sqlite-crypt
Merge pull request #972 from tikhon/mac

Update mac configuration