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. An other option would be to simply use the binary SHA1. But having an ASCII encoding is nice in debugging and avoids 0x00 bytes, for example. So I kept that.