Adds jumping to previous and next changes.

- Adds two new commands: "git_gutter_next_change" and "git_gutter_previous_change"
Adds default windows keymap.

- ctrl+alt+j is already used for the Pretty JSON plugin.
- uses ctrl+shift+alt+j and ctrl+shift+alt+k to align with vim up and down.
Updates readme for keymap info.

- Adds sentence in beginning.
- Adds default bindings.
Refactors.

- Region names as keys mapped to icon names.
Cleanup.

- Removes need for except StopIteration.
Make changes to @DavidLGoldberg's pull request

Put the change command into its own file, dry it up
Add default OSX keymap
Make lines over 80 chars PEP-8 comply
Make lines jumps only jump to blocks over changes

Instead of every line in a block of modifications which is tedious
Make jumping between changes loop back around