Implement `Client.battery();`

Returns `Client._lastBattery`.
Request navdata by default
Add takeoff/land callbacks

Closes #13
0.0.6
Fix uncatched errors when failing to spawn ffmpeg

Fixes issue #44
Merge pull request #45 from eschnou/fix_44

Fix uncatched errors when failing to spawn ffmpeg
Skip broken test

See discussion on issue #47
Merge pull request #48 from eschnou/fix_tests

Skip broken test 'sends config command 10 times'
Maintain a single png and tcp stream in client

Introducing two new API calls: getPngStream() and getVideoStream().
Multiple calls to these methods return the same objects. The underlying
TcpVideoStream is the same and its lifecylce (connect/reconnect) is
managed by the client.
Merge pull request #46 from eschnou/refactor_client_getstream

Maintain a single png or tcp stream in client. 
Closes #43
Added an example on saving the video-stream
Added calibrate command to UdpControl and Client, which according to
the SDK docs "asks the drone to calibrate the magnetometer (must be
flying)."
Merge pull request #26 from janl/battery-levels

Implement `Client.battery()`
Merge pull request #51 from wiseman/add_calibrate

Added calibrate command to UdpControl and Client
Added documentation of client.calibrate to the README.
0.1.0
Merge pull request #52 from wiseman/master

Added documentation of client.calibrate to the README.
Merge pull request #50 from eschnou/save_video_example

Added an example on saving the video-stream
'cb' -> 'callback'
Merge pull request #56 from adrianparsons/master

call a callback a callback