Implement C++ wrappers for libuast

[dennwc]

PR implements a C++ wrapper for C libuast library to simplify the process of writing bindings for languages that can bind to C++ (like Python).

List of wrapped types:

• Role
• Node
• Iterator
• Context

The code still needs a proper memory management.

[dennwc]

It's now ready for the final review :)

[juanjux]

LGTM, I only miss some docs or maybe examples in the README.

[dennwc]

Let's open a separate issue for providing more docs and examples. I want to postpone it because interfaces might change a bit while Scala is in the development.

[juanjux]

Merging, then.