Licensing update

[Brian Anderson]

In light of the press coverage today, we decided we needed to clarify
Servo's licensing immediately.

The core of Servo is MPL2. Many of the peripheral projects are dual
ASL2/MIT, following the Rust license.

Here's what I've done today.

In all servo repositories that are not simply forks of an upstream
project I have done one of two things:

In some projects I've added a LICENSE file containing the text of the
MPL2. The main repository, rust-azure, rust-cairo, rust-css, rust-mozjs
are licensed this way, either because their upstreams are MPL or because
they are core parts of servo.

To the remaining repositories without preexisting licenses I've added a
COPYING file containing the following text.

```
Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
<LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
option. All files in the project carrying such notice may not be
copied, modified, or distributed except according to those terms.
```

Plus the LICENSE-APACHE and LICENSE-MIT files. This is the same
licensing as Rust.

Next I'll contact non-moco contributors to all repos and get them to
send me an email explicitly licensing their code under the relevant
terms. Then I will add a tidy script to our build process that verifies
that all source files have the proper header, then add all the missing
headers.