Improve Server API for transferring sensor data

Improve Server API for transferring sensor data

- Specify format as JSON, TSV or CSV: http://localhost/api/sensors?id=0&format=tsv
  - Default is JSON
  - Currently lower case; make case insensitive?

- Specify number of points (most recent) to transfer: http://localhost/api/sensors?id=0&points=all
  - Default is SENSOR_QUERYBUFSIZ (10)
  - "all" - Will transfer *all* points
  - Currently incompatable with the time seeking options (TODO: Make compatable?)

- Specify time range to return data points: http://localhost/api/sensors?id=0&start_time=5&end_time=6
  - Default behaviour is to just return most recent X points; see above
  - Currently no options for relative (eg: last 5s) time arguments (TODO: Include relative times?)\

Consider:
  - Remove critical sections by using one FILE* for writing and one for reading
    - A critical section is needed around every call to fseek, because the writing thread may write suddenly to the wrong position
    - Will having 2 FILE* mean file can be written and read independently?
      - Need to be sure that this is actually thread safe, otherwise it is more work for no reward
  - Need to simplify Sensor_Handler
    - Added some helper functions, but probably need more
  - Need to do some testing of this API in a test GUI