Added support for the ukhas.net API

Supports submitting received packets to the UKHAS.net API, as detailed in:
https://www.ukhas.net/wiki/uploading_packets

To do so, set the gateway ID using `#define GATEWAY_ID` in the .c file, build, and use the -w switch.

**Requires libcurl** (Ubuntu etc: `apt-get install libcurl4-openssl-dev`)

This is perhaps not yet as neat as we'd like, please review changes before merging.

Suggestions:
* Use the command line switch to specify the gateway id? Like "-w MYNODEID"?
* HTTP response from server should be parsed, retry on failure

Discussion/suggestions welcome :)