Merge pull request #26 from ctismer/master

Add a first Mac OS X installation section #25
remove ambiguous note, because OS X contains no Qt.

Change-Id: Ic58ed0bd56fbc6c763fb48ca7aa4be760a65748d
add dollar sign to commands

Change-Id: Ief7f3eff50e199b008e655d24c256e7d00df555a
remove an old option from virtualenv

Reason:
- new users get confused by options that are gone from official virtualenv
- old users with older versions know the old options, already.

Change-Id: I5c09660995cb5b45468b887dc82b68d6150a83c8
add notes on defunct OS X options

Change-Id: I3debd50e2c3be0b4365faf09db737a439ab9c8f1