Added a simple --upgrade_all argument

[SamuelMarks]

Trivial—naïve?—implementation of an --upgrade_all argument [+ some very minor refactoring (DRY)]

Issue reference: #59
SO reference: http://stackoverflow.com/questions/2720014/upgrading-all-packages-with-pip

[qwcode]

#59 is the main issue now for upgrade changes
the tentative plan atm is to add new support with new top-level commands, not more pip install flags. also note that #59 mentions doing #988 first, before more upgrade changes