Depend on HTML to text module

Simplenews could depend on HTML to text module to do HTML to text conversion, since: a) that module already does the job and b) that's exactly how this is going to work in the D6 port of Simplenews.

The Mime Mail module is getting ready to do this too. By proceeding this way, the HTML to text conversion task gets concentrated on the HTML to text module. Otherwise, as happens today, one with both Simplenews and Mime Mail modules installed has two versions of some HTML to text conversion code.

The HTML to text module (as well as the corresponding D6 original code) is doing a good job. I could say it does, in some degree, a better job than the actual Simplenews and Mime Mail codes. It still needs some improvement, of course, but let the team working on this code take care of it, with the community help, while Simplenews and Mime Mail are concentrated on their specific tasks.