Change ZNC license to Apache 2.0

The following people agreed with the change, in alphabetical order:
(people who approved in several ways are listed only once)
By email:
- Adam (from Anope)
- Austin Morton
- Brian Campbell
- Christian Walde
- Daniel Holbert
- Daniel Wallace
- Falk Seidel
- Heiko Hund
- Ingmar Runge
- Jim Hull
- Kyle Fuller
- Lee Aylward
- Martin Martimeo
- Matt Harper
- Michael J Edgar
- Michael Ziegler
- Nick Bebout
- Paul Driver
- Perry Nguyen
- Philippe (cycomate)
- Reuben Morais
- Roland Hieber
- Sebastian Ramacher
- Stefan Rado
- Stéphan Kochen
- Thomas Ward
- Toon Schoenmakers
- Veit Wahlich
- Wulf C. Krueger

By IRC:
- CNU
- Jonas Gorski
- Joshua M. Clulow
- Prozac/SHiZNO
- SilverLeo
- Uli Schlachter

At https://github.com/znc/znc/issues/311 :
- Alexey Sokolov
- Elizabeth Myers
- flakes
- Jens-Andre Koch
- Jyzee
- KindOne/ineedalifetoday
- Lee Williams
- Mantas Mikulėnas
- md-5
- Reed Loden

At the last few pull requests' comments:
- Allan Odgaard
- Jacob Baines
- Lluís Batlle i Rossell
- ravomavain
- protomouse

The following commits' authors didn't respond:
Trivial changes:
- f70f1086fd0c15d7fdb9eeef95dcefe9781ac3ab
- 4ca8b50e45f3ce77f22f209979fd092b89e7d335

The changes which are not presented in master anymore:
- 5512ed2ea061b2951ae9e24e89537f8ccb021944
- 960a4498f7b04f652462dfbff131a4d7c4701fdb
- 0f739de2c083bf19c7e4ee080390a4840ccecbee
- 7f53cc810b886351c452652cc32cea9b4cca1a8c

Fix #311
Fix #218