Pre flight check

Base framework for Flight Checklist
- Customizeable via XML. Current release only support XML in resource/xml
- using ExpandableListView grouped in 2 categories
 1. User checklist
 2. System checklist
- categories are not hardcoded - simply edit the XML file to add categories
- no listener yet
- not linked to drone variable

Todo next...
1. Add listeners to the row
2. link to drone variables (batt level, GPS3D etc)
3. load custom checklist from SD card
4. refactoring for other use?

Note: this is work in progress....

some screenshots:
![screenshot_2013-10-18-20-01-21](https://f.cloud.github.com/assets/5212844/1360051/c2a185d4-37ed-11e3-9d1a-a8e04f59394b.png)
![screenshot_2013-10-18-20-01-58](https://f.cloud.github.com/assets/5212844/1360052/c752ad6a-37ed-11e3-8e42-1ea7c2229613.png)
![screenshot_2013-10-18-20-02-11](https://f.cloud.github.com/assets/5212844/1360061/0074c7ae-37ee-11e3-964a-badd9a81af40.png)