Networking: added function and cleaned up #14

This PR adds a new function network_is_connected which can be used to verify the status of the connection before executing something which needs network access. It changes the boolean is_connected for an atomic one, in order to be thread safe.

It also removes a redundant include of EthernetInterface, and replaces it with an include of the NetworkHandler header.

Looks great!

Networking: added function and cleaned up