Total revamp of readline support.

Now using plain C bindings to readline, instead of this super messy ffi+coroutine binding.

The completion system is now in C, taken from the one we have in LuaJIT.

The line parsing, history, multiline support, support for "no =", is still implemented in Lua.

Notes/TODO:
* multiline works, but pasting code results in double prints (not sure how to fix that)
* there's no readline.so detection, it just assumes the lib exists (we should add cmake detection, anyone interested?)

This new version seems to fix problems reported in [this issue](https://github.com/torch/trepl/issues/11).