Introduce LXC monad.

Changelog:
- added `mtl` library as dependency
- introduced `LXC` monad as newtype wrapper for `ReaderT (String, Ptr C'lxc_container) IO`
- convert container-related functions into `LXC` monad
- added `C'lxc_container` marshalling helpers (e.g. `withC'lxc_container`)
- `listContainers` functions now return `[Container]` instead of `[(String, Ptr C'lxc_container)]`
- removed `mkContainer` function
remove getRef and dropRef from System.LXC.Container
reduce Foreign imports
encapsulate LXC monad

Changelog:
- `LXC` now also has instance of `MonadIO`
- `LXC` constructor dropped from `System.LXC.Container`
- `withContainer` now acts in any `MonadIO`
- `transformers` package added to dependencies
add Show instance to Container data
added example
updated README Usage section
added comments to example
tiny README fix
fix possible segfaults
added more explanation to Usage section
Merge pull request #1 from fizruk/lxc-dsl

LXC monad