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
- description is ready but not implemented yet

Todo next...
1. Add listeners to the row
2. link to drone variables (batt level, GPS3D etc)
3. add checklist description on long touch of item
4. load custom checklist from SD card
5. refactoring for other use?

Note: this is work in progress....


this is how the XML file looks like:

```
<?xml version="1.0" encoding="utf-8"?>
<preflightcheck
    type="APM"
    version="1" >

    <category label="User Checklist" >
        <check_item
            description="Ensure Camera is connected and switch on"
            mandatory="yes"
            title="Camera connected and switch on" />
        <check_item
            description="Ensure surface control is in the right direction"
            mandatory="yes"
            title="Surface control - Aileron" />
        <check_item
            description="Ensure surface control is in the right direction"
            mandatory="yes"
            title="Surface control - Elevator" />
        <check_item
            description="Ensure surface control is in the right direction"
            mandatory="yes"
            title="Surface control - Rudder" />
    </category>
    <category label="System Checklist" >
        <select_item
            description="Select connection type"
            mandatory="yes"
            optionlist="Bluetooth,USB,WiFi"
            selectindex="0"
            system_tag="SYS_TELEMETRY_TYPE"
            title="Telemetry connection :" />

        <toggle_item
            description="Toggle sytem arm or disarm"
            editable="yes"
            mandatory="yes"
            system_tag="SYS_ARM_STATE"
            title="Arm System" />

        <radio_item
            description="Select connection state"
            editable="yes"
            mandatory="yes"
            optionlist="Connect,Disconnect"
            selectindex="0"
            system_tag="SYS_CONNECT_STATE"
            title="Telemetry connect" />

        <value_item
            description="Show battery level"
            editable="no"
            mandatory="yes"
            minimum_val="3.5"
            nominal_val="3.7"
            system_tag="SYS_BATTERY_LVL"
            title="Battery level" />
    </category>
</preflightcheck>
```

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)