Update BIP-0001 for github move

- Mention github instead of wiki
- Add markdown as allowed format
- Fix some weirdnesses such as "A BIP editor must subscribe to the gmaxwell@gmail.com list.". Remove the repetition and create a new section which lists the BIP editors (currently only gmaxwell).
- Prefer discussion on mailing list to forum

This is just a cleanup and change for the github move. I have not changed the process to jgarzik's new proposed process (http://sourceforge.net/mailarchive/forum.php?thread_name=CAAS2fgROsymXnXTrfLTLTQ%3DEDwaAFu%2BxrrD4Q-Gye5XWze7vFw%40mail.gmail.com&forum_name=bitcoin-development), this can be done in a later separate change.