Killed 2 out of 3 mbstring uses.
Different approach to counting UTF-8 string length
Only convert code points within planes 0-2

Before 82421eb, the mb_decode_numericentity function wouldn't convert code points beyond plane 2 (per the flags).  This change essentially adds the same behavior.
Case folding support for environments without mbstring

Per the spec, reference link matching is done by normalizing the label with a Unicode case fold, which mb_strtoupper provides.

But since not all systems have the relevant extension installed, we need to manually implement this logic by converting characters according to this table: http://www.unicode.org/Public/UNIDATA/CaseFolding.txt
Change mbstring requirement to a suggestion
Update changelog
Make scrutinizer happy
Merge pull request #40 from thephpleague/rm-mbstring

Remove mbstring as a dependency