add perference header for legacy preference
use custmoized PreferenceFragment for pre 3.0 compatible

fix haiwen/seadroid#124
optimize Gesture Lock
add backward compatibility support

1. create a new package "com.seafile.seadroid2.gesturelock"
2. add Chinese translations
code cleanup
remove hard code strings

translate Chinese strings
add Gesture Lock layout screen size adaptation
code cleanup

close #124
code add paddings to Gesture Lock layout
change text color for button
update README and pom files
code cleanup
code cleanup
update Utils
add perference header for legacy preference
use custmoized PreferenceFragment for pre 3.0 compatible

fix haiwen/seadroid#124
optimize Gesture Lock
add backward compatibility support

1. create a new package "com.seafile.seadroid2.gesturelock"
2. add Chinese translations
code cleanup
remove hard code strings

translate Chinese strings