Update README.md

Environment variables section.  First entry is for overriding the drone IP address to be used inside the drone simulator

If your variable makes it into master make sure this section gets updated. 

See [pull request](https://github.com/felixge/node-ar-drone/pull/12#issuecomment-9632230) for more info 
Implement `Client.battery();`

Returns `Client._lastBattery`.
Merge pull request #14 from csanz/patch-1

Update README.md - Added Environment Variables Section
update to latest buffy (performance)
version 0.0.5
Update README.md

The duration parameter for client.animate('flipleft') is now   milliseconds exceeding .015.
Merge pull request #39 from MarcusSorealheis/patch-1

Update README.md
Document how to access the bottom camera, fixes #35

Thanks @seken for that hint
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()`