Support arbitrary length bit strings

[mdeshon-google]

• Added second EncodeString prototype using vector for >32 bit strings (byte-aligned)
• Added second MakeBloomFilter protoype using vector for >32 bit strings
• Converted rappor::Encode to throw exceptions instead of using assert()
• Wrote unit tests, including one to ensure that old and new EncodeString work equivalently
• Still throw errors if attempting to use HmacSha256 with >32 bits