Check list

This is a new pull based from the previous preflight checklist #355 

Base framework for Flight Checklist

Customizeable via XML. Current release only support XML in resource/xml
using ExpandableListView grouped in 2 categories
User checklist
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?~~ 

Connection to DP internal System variables:
1. ~~SYS_BATTREM_LVL -  (R) Battery remainder level %~~
2. ~~SYS_BATTVOL_LVL -  (R) Battery voltage level V~~
3. ~~SYS_BATTCUR_LVL -  (R) Battery current level A~~
4. ~~SYS_GPS3D_LVL -  (R) GPS 3D satellites~~
5. ~~SYS_DEF_ALT -  (R/W) default Altitude~~
7. ~~SYS_ARM_STATE - (R/W) Arm/Disarm state~~
8. ~~SYS_FAILSAFE_STATE - (R) FS State~~
9. ~~SYS_CONNECTION_STATE - (R/W) Telemetry connection state~~
10. SYS_CONNECTION_TYPE - (R/W) Telemetry connection type

New Item Type to be added:
1.~~TextNote - suggested by @ThorstenDP~~

Note: this is work in progress....

The code has been refactored to support future works
Layouts are also been improved
![screenshot_2013-10-21-01-24-15](https://f.cloud.github.com/assets/5212844/1368559/bd48a3ee-39ad-11e3-9200-043a887746e5.png)