Update Credits for 2.1

+ Adds Corporate Team Section
+ Adds SMF friends and GitHub Contributors to Special Thanks
! Changes 2.0 to 2.1 :P
Update Colin's name
Few updates

Includes 2.0, merges current and former in alphabetical order. More
suggestions added to FTC.
- Motoko-Chan, + Spuds

Motoko-Chan has requested not to be listed, and Spuds being added
against his will! >:D (Confirmed by PM to "do what you feel is
appropriate" and he damn well deserves to be listed.)
Correctly naming Site and Server Team

http://www.simplemachines.org/community/index.php?topic=511093.60
Correcting my derp English

Signed-off-by: Acans <acans@simplemachines.com>
Add contributors.txt file

Signed-off-by: Acans <acans@simplemachines.com>