New TCP and UDP socket APIs

This pull request contains new socket-like APIs for UDP and TCP, as discussed in https://github.com/contiki-os/contiki/issues/527. Historically, it has been really difficult to write applications in Contiki that use UDP or TCP to communicate. As a developer, you had to be intimately familiar with numerous implementation details. These new APIs are intended to reduce this friction.

The APIs are documented in their respective header files.