Fixup of the authors:
Moved Matt to the credits section as he doesn't actively work on kdevelop4 source code.

Separated credits into two sections, the first part lists people that contributed to kdevelop4, the rest is separated by an "old contributors" entry and lists anybody who has been in the list but worked only on kdev3.

If anybody reading this feels he should be
a) moved from author to credits or vice versa
b) added to either of the sections as he isn't listed just let me know by replying to this mail.