Adds browse command to `pod spec COMMAND` family to open the homepage of a given spec

Requested in #1527

Example usage: http://www.youtube.com/watch?v=2XPtSKVB_50

I think testing the actual functionality of this (opening a browser) would be pretty hard, so I didn't do that.

Checking for the existence of the podspec and for ambiguity seemed appropriate, though, so I pulled that behavior out from the `pod spec which` tests and into common methods that `which`, `browse`, `edit`, and `cat` could all call.