Relax version constraint on text
Mention what a crit-bit tree *is* (thanks, reddit!)
.gitignore: ignore Cabal sandbox dir and config file.
Formatting.
Small tweaks to 'maskLowerBits'.

Intention is to make it more clear what is going on.
Clarify a comment.

From my reading of the code, this is how things actually work.
Trailing whitespace.
Merge pull request #11 from 23Skidoo/small-fixes

Small fixes
Implemented filter