Fix hyphenated ID decoding.

Decoding a hyphenated ID should no longer cause an index out-of-bounds
panic. Add tests for `DecodeString()`.
`MustGenerateIds()`: Remove redundant error check.
Simplify `GenerateBytes()`.
Merge pull request #64 from mozilla-services/bug/decode-hyphenated-id-redux

Fix hyphenated ID decoding, take II