Accept timezone as string

[floatdrop]

This gives no significant performance boost, but code will a little bit cleaner. For example:

import {CivilTime, convert, tz} from 'cctz';

convert(new CivilTime(2015, 9, 22, 9), tz('America/Los_Angeles'));
// vs
convert(new CivilTime(2015, 9, 22, 9), 'America/Los_Angeles');