Methods to serialize JSON with non-ascii characters escaped

In some situations (such as JSONP with strings containing U+2028 or U+2029) it is useful
to escape all non-ascii characters in the generated JSON string.
This provides Json.asciiStringify and JsValue.toAscii to accomodate this.
Remove JsValue.toAscii
Merge pull request #2311 from tmccombs/escapeNonAscii

Methods to serialize JSON with non-ascii characters escaped