Enhance string hash

This tackles both issues mentioned in #24:

1. It increases the base length from 30 to 36.
2. It uses the full sha1 hash instead of only half of it.

It avoids wasting to much characters by using base64 instead of hex encoding for the sha1 hash.
Replace constant in MonolingualTextHandler
Merge pull request #44 from wmde/stringhasher

Enhance string hash