Fixed ST2 errors

As discussed in #2, Sublime Text 2 would be appreciated. This PR revisits and fixes up issues that ST2 was having. In this PR:

- Move from `add_all` to `for + add` to add lists of regions
    - ST2 only supports `RegionSet's` for `add_all` which ironically cannot be instantiated in Python
    - http://www.sublimetext.com/docs/2/api_reference.html#sublime.RegionSet
- Moved to `Region` constructor for inverting regions
    - ST2 did not like setting `a` and `b` properties of an existing region (I guess they are immutable)
    - http://www.sublimetext.com/docs/2/api_reference.html#sublime.Region

/cc @philippotto