Add Apache-2.0 and Zlib license options.

[Lokathor]

This PR adds the "rust standard" Apache-2.0 license as a license option, as well as the Zlib license because it's even more permissive than the MIT license. The MIT license is of course also still an option.

Being a change in license, this will need the following folks to agree:

• Osspial
• Didgy74
• ryanisaacg
• repi
• msiglreith
• dxenonb
• aloucks
• LeshaInc
• Dispersia

Closes #41

[Lokathor]

And a ping to:
@Osspial
@Didgy74
@ryanisaacg
@repi
@msiglreith
@dxenonb
@aloucks
@LeshaInc
@Dispersia

[msiglreith]

Thumbs up enough :D?
otherwise,

I license past and future contributions under the 'MIT OR Apache-2.0 OR Zlib' license, allowing licensees to chose either at their option.

[aloucks]

I agree.

[dxenonb]

I approve of the change.

[Didgy74]

I approve.

[Lokathor]

@Osspial On the grounds that the only outstanding vote is from @LeshaInc, who seems to have disappeared from github without a way to email them a follow-up, and also since they only added the Hash derives, I think that you should now merge this.