Multiple comment provider support (#606)

This branch implements support for multiple alternative comment providers:

- Disqus
- IntenseDebate
- LiveFyre
- Moot

This is completely backwards compatible:  Custom themes will continue to work (supporting only Disqus)
The changes required to make custom themes support this are completely documented in the porting-to-v6 guide (and are easy).

Also, documented the new options in the manual, and deprecated the DISQUS_FORUM option because it was obviously inappropiate for the new possibiities.