In order to play a bit more nicely with Android, IDs are now ints
instead of longs. At 500 loads/second, it would take an int 49 days to
wrap. So this seems reasonable. Additionally, this means that one can
use SparseArrays and resource IDs without casting.

Deprecated markers have been placed on the old methods.

In this implementation, trimming is done based on when the item was
written to the cache.

In order to make ImageCache as easy to use as possible, this adds a
feature to automatically trim the disk cache in order to satisfy a given
storage size. By default, this size is determined automatically as a
fraction of the available free space. Cached items that haven't been
accessed by the given ImageCache instance will be purged first, followed
by items that haven't been accessed recently.

All the parameters of autotrimming can be configured, including the use
of autotrim at all.

Closes mitmel#8