Raise FatError when a step fails.

Refs #2.
Goodbye Fat#fetch_at.

Refs #2.
Updating benchmarks

Refs #2.
Improve performance when failing.

Refs #2.
Add test for empty hash.

Refs #2.
Include the hash we're looking into in the error message.

Refs #2.
Update README.md.

Refs #2.
Merge pull request #2 from tonchis/2-at-raises-error

Should we return nil when finding a non-hash value down the path?