Added accelerometers to all platform devices

Only available if the device supports it, otherwise it will be null on
the IDevice instance.
Add screenshots for WP8.0
Merge pull request #35 from saramgsilva/master

Add screenshots for WP8.0
Accelerometer page added

Needs custom renderer for WP8, iOS & Android are implemented. Fixed
Android bug on accelerometer by dividing the output by 9.81m/s^2 to
align it with iOS & WP8.
Merge pull request #5 from XForms/master

Merge from main