Change license to Apache 2.0

[chfast]

A proposal to change license from MIT to Apache 2.0.
This is mostly to be consistent with other projects we created recently.

I need approvals from @axic @ehildenb and @gumb0.

Closes #60.

[ehildenb]

@chfast it looks like you're supposed to include this entire block of text at the top of each file:

   Copyright [yyyy] [name of copyright owner]

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.

   You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.

[chfast]

I'm ignoring this, too much lines.

[chfast]

I prefer 2 lines only. I think it is quite clear what the license is. I've see in it some Google projects:
https://github.com/golang/go/blob/master/src/sync/map.go#L1-L3

[axic]

I am still not sure which is better for this project, but I do not mind either.

[chfast]

@ehildenb Are you ok with this?

[ehildenb]

Seems like we should include the full text that the license assumes, but the switch seems ok too.