handle surrogate pairs as single char in string functions

[andrew-coleman]

Codepoints outside the Basic Multilingual Plane are represented in Javascript by 2 characters (surrogate pair). This causes $length('💩') to return 2 rather than the expected 1.

$length, $substring and $pad have been fixed to produce the expected results for strings that contain these codepoints.

Resolves #156

[coveralls]

Coverage remained the same at 100.0% when pulling e6ff177 on surrogate_pairs into 2010889 on master.