New contribution guidelines

[David Capello]

Hi there,

Now there are new guidelines for people that want to contribute.
Basically three things:

1) Now the official branch for development is 'master'

2) The branch 'dev' doesn't exist anymore

3) A modern compiler with some C++11 support will be required

   (MSVC 12, and Xcode 5.1.1)

INSTALL.md and CONTRIBUTING.md files are updated with these new instructions.

Also the Linux port is not compiled regularly, so its compilation could fail.

- David