Update blame view upon save

Fixes #29.

Test plan:

First test editing when view is open:

	* Open blame
	* Add some newlines
	* Save file
	* See blame view update

Now test it updates upon second display:

	* Open blame
	* Close blame
	* Add some newlines
	* Save file
	* Open blame again
	* See it update
Merge branch 'master' into update-upon-save
Actually update blame view upon save

In the last commit I think I simplified the code and tricked myself into
thinking I tested it, but I must have not reloaded the package. This commit
works though.

Test plan:

First test editing when view is open:

 * Open blame
 * Add some newlines
 * Save file
 * See blame view update

Now test it updates upon second display:

 * Open blame
 * Close blame
 * Add some newlines
 * Save file
 * Open blame again
 * See it update
Fix default date format string at startup

Not sure what happened when I tried to do this in 39133b1, but evaluating
`DEFAULT_DATE` at import time doesn't work because `atom.config.get` returns
undefined. So calculate it later and cache it.

Test Plan:
Reloaded window then opened blame in a big file. Saw my preferred datestring
and not the full ISO 8601 format.
Merge pull request #31 from dmnd/update-upon-save

Update blame view upon save