Add support for RegExp separators to String#split

[ndreynolds]

Add support for RegExp separators to str_proto_split in String.c and new tests in test_string_global.js.

Currently, only a string can be given as the separator, but the spec also allows a RegExp, e.g.

"Hello 1 word. Sentence number 2.".split(/(\d)/)
// ["Hello ", "1", " word. Sentence number ", "2", "."]