Firefox 18+ supports String#startsWith, String#endsWith
Optional position argument

Added optional position argument to `String#startsWith`, `String#endsWith`, as described in https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/String/startsWith and https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/String/endsWith.
Documented new arguments and examples.
Updated unit tests
Merge pull request #96 from victor-homyakov/patch-33

Use native `String#startsWith` and `String#endsWith` if present. Turn existing implementation into a true polyfill.