Retrieve Gravatars by mime-type

Old avatars will eventually be purged by the delete method
Removed magic string
Merge pull request #1571 from BarryThePenguin/avatar-extensions

Retrieve Gravatars by mime-type Fixes #1467
ui avatars: Gravatar also supports GIFs
avatars: Fix redownloading of avatar every time
repo: Fix crash due to last timestamp being empty when adding a new empty project
eventlog: Add todo
mac setup: Fix font on error page
fetcher git: Configure line endings correctly. Fixes #1568
extensions: Timestamps < 24h ago can also be yesterday
Removed P/Invokes, now uses native ContextMenu handling.
Use LastWriteTime instead of CreationTime
Merge pull request #1574 from BarryThePenguin/patch-1

Use LastWriteTime instead of CreationTime
Merge pull request #1567 from Silvenga/master

Improved Icon Menu Handling
plugins: Spell GitHub properly
avatars: Fix #1576
avatars: Use more reliable LastWriteTime to determine whether the avatar needs refetching
Added new icon for Mac Yosemite+
Merge pull request #1586 from snwh/master

Added new icon for Mac Yosemite+
mac ui: Integrate new Yosemite folder icon. Closes #1585