Fixes for #17 to speed up ORDER BY and OFFSET on key and index columns

Here are a few changes to speed up SQL queries that use ```ORDER BY``` and ```OFFSET``` on the ```key``` and the index columns.

1. SQL INDEX are added for the ```key``` and ```inc``` columns. If the ```inc``` column is never sorted by, its index should not be added.
2. ```key``` and ```index``` columns have been moved from ```BLOB``` to ```TEXT```. This should speed up ```ORDER BY``` even more. I'm not familiar with how this will affect collation, if anything, but it's probably no worse than it is already.

These should fix #17.