Clean up the exisiting Pluggable Transport spec

The current Pluggable Transport spec has gotten more and more Tor specific, which is bad for people that aren't Tor that wish to use the managed PT API. The current spec should be cleaned up to address:

• Why managed mode is a good idea.
• Include the ExtORPort stuff.
• Make it abundantly clear that not just Tor can/should use this and managed mode.
• Other misc. spec clarifications as needed.

Note: This is entirely separate from writing a v2 of the Pluggable Transport spec that fixes the warts in the existing one (which will involve code).