Support unicode escape sequences in PHP 5.6

[thekid]

Implements #38

$str= "\u{1F602}"; // 😂

[thekid]

Implementation notes on html_entity_decode, which isn't the fastest, but:

• Using json_decode() as suggested here doesn't work for \u{1F602}. JSON only supports \uXXXX, and we would need to manually split the character into two parts, requiring a conditional and some string padding calls.
• Using iconv() or mb_convert_encoding() in conjunction with pack("H*") and then using UTF-16BE suffers the same problem, working fine for e.g. \u{00F1}, but not for the one above.