[doc] A more complex tutorial. Still a few issue to handle.
[compiler] Allows let expression in end block of recipe
[compiler] Remove support for ruby wrapper through swig

It is not working since ages. Moreover, it is not a good idea to add
another code generator in the loop :), we have all the needed
information in hyperc.
[network] Make proxy_serializer::eval return a boost::optional

Be consistent with local_proxy::eval, and no need to reimplement it
badly.
While here, remove some unused headers