Solidity function AST nodes get documentation attribute

Solidity AST nodes get the documentation string attribute if the comment above the function matches the NATspec format (3 slashes for now).

I am not really sure I like the fact that having an empty documentation string and having no documentation string can't be differentiated at the moment. Perhaps this should be fixed. Comments are welcome.
We certainly can, but we will also have to move some inline functions implementations to the cpp file.