This allows the `Editor` to implement `Sync` as long as it contains a completer which implement `Sync`. It lets the `Editor` take ownership of the completer.

Building and testing all the different windows environments against
stable/beta/nightly would take too long.  Reducing the environments msvc &
gnu 64 bit environments for beta and 1.9.0 rust

Fixes #18

Supported only on Rust Nightly due to unstable features