MOSYNC-3273: Documentation for CustomPicker widget

Documentation for CustomPicker widget: http://jira.mosync.com/browse/MOSYNC-3273

There is also a commit for DatabaseTest documentation in this pull request: http://jira.mosync.com/browse/MOSYNC-3280 It is not completely done yet, but it is in a shape that can be merged. However, more updates will follow on it (new screenshots).