Handle multiple wheel versions

[joshclimacell]

If there are multiple suitable versions, pick the alphabetically largest one.

Also:

• Provide option to delete downloaded wheels that are no longer used
• Make pip_generate_wrapper globally visible
• Better error messages for duplicate wheels

[joshclimacell]

@evanj We updated our fork with these changes a while back; thought I'd try to get this into the main repo in light of #43...

[joshburkart]

Ping @evanj! Also, should we consider setting this up in its own project, and having at least a couple people who can accept PRs etc.?

[joshburkart]

Ping @evanj.