Include SeekToFirst and SeekToLast.

Following the LevelDB iterator protocol
(http://leveldb.googlecode.com/svn/trunk/doc/index.html), I have
added ``SkipList.SeekToFirst`` and ``SkipList.SeekToLast`` support.

The tests and documentation have been updated to reflect this.
Merge pull request #3 from matttproud/feature/seeking/beginning-proximate-end

Include SeekToFirst and SeekToLast.