Use gutter instead of manual marks.  Resigned rename command.

The gutter is now used to mark files which I think people would expect
and it automatically gives us color (somehow).

The rename command now allows you to edit filenames directly in the
buffer instead of being restricted to a single file.

(These should have been separate commits, but I was part way through
rename before I thought of using the gutter.)
Smarter initial directory

Closes #3.  The initial directory is the first of:
 * The current view's directory.
 * The project's first folder.
 * The user's home directory.