The imageproxy command no longer uses the in-memory cache by default.
Instead, no caching is enabled by default and the in-memory cache can be
enabled by passing `-cache memory`.
The -cache flag also supports specifying directories, and therefore
replaces hte older -cacheDir flag. cacheDir is still supported for
compatibility, but is deprecated and will be removed in the future.
This also sets us up to add support for additional caching backends in
the future.
Partial fix for #49.
These were originally added to prevent a denial of service caused by
requesting very large images, but are no longer needed since we no
longer resize images larger than their original size.
