caching avatars using urls, not userId

Closes #349
storing avatars in a separate folder for user

All avatars for a user are stored in a separate folder with the
user id as the name. All files in that folder are deleted before
writing a new one if the avatar doesn't already exist in that folder.
This should solve the problem with the old avatars when
avatar url is changed.
Remove unused parameters in CommitDiffListFragment

Remove unused parameter from private method,
reduce scope of local variables.
Closes #387
added a posibility to open commits compare screen for pull-requests. issues screen now has 'commits' button
commit icon near to 'commits' field
moved 'commits' label below the description
Use ViewFinder to set text on TextViews
Merge pull request #347 from atermenji/pr_commits

Commits list for pull-requests