mavconn: Move MAVConnSerial to libev.

Adds stub for open URL function.

Issure #54.
mavconn: Move MAVConnUDP to libev.

And fix docs in serial.

Issue #54.
package: Add temporary rosdep for libev-dev.

Issue #54.
Add libev to README
Merge pull request #56 from vooon/54_try_libev

Switch to libev