add avatar load service
modify avatar load service
save avatar info to loacl db

1. create a new package "com.seafile.seadroid2.avatar"
2. uniform database helper class name to be "**DBHelper"
3. support displaying avatars at off-line
fix load avatar algorithm
fix SSLHandshakeException

1. trust every server - dont check for any certificate
2. create a trust manager that does not validate certificate chain
code cleanup
avoid tons of db insert request
use cached SSLSocketFactory

use self keystore trust manager rather than trust all host

reference nostra13/Android-Universal-Image-Loader#240
delete "is_default" column from table "Avatar"
trust all host when load avatars
Merge pull request #168 from Logan676/avatar

load and display account avatars