Adding notes to each serial case

This changeset adds comments to outline the format expected from each of the serial send and receive commands. A few of the options seems inconsistent in this changeset but haven't been fixed (e.g. gps expects 9 data points if gps is enabled, but 6 if not).
Changing serial commands to make the various ifdef options more comparable.

A few of the serial commands (gps, mag) were not the same for the different IFDEF options available. This changeset attempts to make these different options more equal so that a programmatic interface to the configuration doesn't need to know as much about the internal configuration in order to read consisten output. Zeros are printed, or dummy values read, instead of the real values where necessary.
Adds code to run the serial communication over XBEE or USB.

This changeset initializes USB and XBEE serial ports on the AeroQuad at the same time, and allows communication over either port. The board will respond on the same port
that received the command. There is a new serial option 'Q' that can be used to configure the XBEE (following page 27 of the XBEE-pro documentation for the commands) and everything is wrapped in a new define "MultipleSerialTelemetry" that is independent of the default option or the WirelessTelemetry option.
Merge branch 'development' into xbee_serial_dual
Merge branch 'development' into serial_comm_labeling
Intermediate commit for cleaning up code to AQ standards
debugging altitude hold; now it should be at AQ 2.4.2 status

* added '^' command to set target altitude
* added '?' command to help debug altitude hold and heading hold
* enabled XBEE (#define WirelessTelemetry)
* switched XBEE baud to 57600
* commented out the AltitudeHoldRangeFinder #define, as we don't have one and it was
  messing up altitude hold
* enabled the battery monitor (although it seems to be controlled by EEPROM values, as it
  doesn't blink the appropriate LED when batteries are low)
Removing OSD defines to save 0.5ms
Merge branch 'aq_makefile' into flighttest1
changing to kinematics_dcm, maybe too slow
Changes for kinematics_dcm to compile
Merge branch 'flighttest1' of github.com:ashima/AeroQuad into flighttest1
Reverting 1c97deb..c4cc43e
Changing gyroscope to 42Hz
Changing accelerometer to 40Hz
Changing battery warning ratio
moving measureCriticalSensors to 100Hz loop, no using 40/42 Hz measurements
Adding logging to openLog at mod 10Hz ==5
Mag averaging code running at 100Hz

Adds new code so that mag data is sampled at 100Hz and averaged during the 10Hz step, rotated and used to calculate the heading.
Merge branch 'serial_comm_labeling' into flighttest1