New `JKParseOptionPermitLeadingZero` option

Normally when parsing numeric values, leading zeros are considered a parsing error. This is the correct behavior, but while assisting an individual diagnose a parsing error due to incorrectly formed JSON (see [this Stack Overflow question](http://stackoverflow.com/questions/16569485/ios-json-parse-return-null-when-called-for-second-times/16569953#16569953)), I suggested that he might be able to adjust `jk_parse_number` to permit leading zeros.

So, I offer this pull request with trepidation (as the correct resolution for that individual's problem is to fix his JSON), but nonetheless I thought I'd share this with you. Feel free to accept, improve, or simply reject/close this pull request as you see fit.