Documentation update for Howjsay

Adds new page to website, drops screenshots from the TTS-API.com (since
it does not have options to configure, like Howjsay), and updates README
for the add-on.
Check for text after service-specific modify()

During processing by the router, if the service instance runs modify()
on the text and yields an empty string, processing should stop at that
point rather than be passed back through to the service.
Quote mapping for util_approx()

Adds fancy quotes to the list of mappings performed by util_approx().
Word improvements for Howjsay

Also allow digits, apostrophes, periods, and at-signs to appear in words
sent to the server, also adding encoding.