Allow for unescaped unicode characters. Closes #4.

This includes a fallback for 5.3 to replace the unicode characters
encoded by json_encode without the unescaped unicode bitmask option.
Added check for mbstring for 5.3 unicode non-escape.

Also added notes about support for 5.3 and a note about the behavior if
mbstring is not installed.

Fixed composer parse error.
Merge pull request #5 from zumba/unescaped-unicode

Allow for unescaped unicode characters. Closes #4.