make ffi/util module local
update README file to clarify how to use make clean
remove global util module
Merge pull request #619 from chrox/master

update README file to clarify how to use make clean
take care of directory that we don't have permission
add ant prerequisite for Android build
Merge pull request #621 from chrox/master

take care of directory that we don't have permission
os.execute may cause FC on Android
font weight option should be persistent between launchings
This should fix #620.
update pot file
Merge pull request #625 from hwhw/master

fail gracefully when the Kobo light device cannot be opened
Merge pull request #626 from chrox/master

font weight option should be persistent between launchings
fix(setting): handle empty setting files properly

empty reader setting evalues to nil
Merge pull request #627 from houqp/houqp-master

fix(setting): handle empty setting files properly