Erbium: client-side support for CoAP Observe

This pull request extends Erbium with client-side support for CoAP Observe.
An example showing how to use the new functionality is also included.

As requested by @mkovatsc, the new code is not compiled by default. To enable it the `COAP_OBSERVE_CLIENT` macro must be defined equal to 1. On a Tmote Sky, this will increase the Text section by ~1100 bytes and the BSS section by ~300 bytes.

Happy Holidays! :)