use handler to load avatars
code cleanup
improve methods name
improve code style
make handler send all avatar info to ui thread

imporve code style
improve runtime efficiency
avoid duplicate inserting to database
catch exceptions
show cached avatars when offline
imporve methods comments
add TODO task
remove handler
improve comments
improve names of methods and class
refactor
refactor again
fix review tickets
Merge pull request #208 from Logan676/refactor_avatar

refactor avatar loading module