changed license from GPL to MIT or BSD

I'd like to propose that you re-license this work to MIT or BSD. The GPL effectively restricts its distribution with *combined works* which refers to code containing GPL'd code.

In a nutshell, here are the constraints of the MIT license (which I've included in this PR):

* Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
* Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

BSD (3-clause version) is similar, adding this notion:

* Neither the name of the <organization> nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

Now, both of these are GPL compatible, but GPL specifies a few additional things:

* Copies may be distributed free of charge or for money, but the source code has to be shipped or provided free of charge (or at cost price) on demand. The receiver of the source code has the same rights meaning he can share copies free of charge or resell.
* The licensed material may be analysed or modified.
* Modified material may be distributed under the same licensing terms but don't have to be distributed.

What do these mean? Essentially, you cannot distribute a library which uses some GPL'd code unless that library, too, is released under the GPL. It would be harder for people to use & release timber as part of an open-source software package.

For example, if Twig were GPL'd, and timber were BSD, you would not be allowed to release timber as a BSD project under the terms of using Twig.

Here is some further reading if you like:

* MIT: http://opensource.org/licenses/MIT
* BSD, 2-clause: http://opensource.org/licenses/BSD-2-Clause
* BSD, 3-clause: http://opensource.org/licenses/BSD-3-Clause
* GPL, v2: http://opensource.org/licenses/gpl-2.0.php
* GPL, v3: http://opensource.org/licenses/gpl-3.0.html 
* LGPL, v3: http://opensource.org/licenses/lgpl-3.0.html