Fix some compiler warnings

When set to warn about everything, LLVM will generate a bunch of warnings some
of which are fixed in this commit.
Add lastVis property to LFSContent

lastVis is the visibility of the previously added content item.
If avatar image fails to load, cache placeholder image instead

Fill the cache slot with placeholder image so we don’t repeatedly hit
the server asking for pony unicorns.
Improve memory usage by not caching images before scaling

Do not cache larger 75px images as they are not going to be shown that often
(only when the user actually taps on a cell to go to detail view).
Merge pull request #48 from Livefyre/dev

Improve memory usage by changing how avatar images are scaled and cached