mavconn: Implement URL parsing.

Supported shemas:
* Serial: `/path/to/serial/device[:baudrate]`
* Serial: `serial:///path/to/serial/device[:baudrate][?ids=sysid,compid]`
* UDP: `udp://[bind_host[:port]]@[remote_host[:port]][/?ids=sysid,compid]`
* TCP client: `tcp://[server_host][:port][/?ids=sysid,compid]`
* TCP server: `tcp-l://[bind_port][:port][/?ids=sysid,compid]`

Note: ids from URL overrides ids given to open_url().

Issue #58.