Add SmartVideo flight patterns and Yaw

Added the 'SmartVideo' programmed flight commands (up-and-out, circle, etc.), all the currently known flight commands are now provided by this package.

Added a calculated Yaw field to the IMU data to allow precise yaw monitoring.

Added QuatToEulerDeg() which calculates roll, pitch and yaw from quaternion data. No need to use this if you only want yaw (see above).

Add some MVO and IMU data

Add parsing of MVO (position, velocity) and IMU (quaternions, temperature) data.
Added a trivial usage example in the README to get people started.

Fix sign of VerticalSpeed

Package now changes sign of vertical speed that the Tello sends in each flight data packet so that rising is positive and falling is negative.

Minor update

DroneBatteryLeft has been renamed to the more accurate BatteryMilliVolts.

Second public release - more functions

This release adds some new functions to the API: GetMaxHeight(), GetSSID(), GetVersion(), GetVideoBitrate(), GetLowBatteryThreshold() and SetLowBatteryThreshold(), SetVideoNormal() and SetVideoWide().

Documentation has been updated and a new ImplementationChart.md gives an overview of the Tello commands we currently understand and how this package handles them. It is intended to complement, not replace, the godoc documentation of the package.

A field in FlightData - BatteryLower has had its name changed to the more accurate BatteryCritical. Some new fields have also been added to this struct to support the new functions above.

Initial release - API subject to change

First public release of my standalone Ryze Tello Golang package.

Documentation and a sample application using SDL is available.