BUG: Use a cache_dir based on the prefixes of Enpkg
Fix egg-collection query when multiple prefixes exist.

For example, if the newest egg was installed in the prefix path,
calling `enpkg --whats-new` listed that egg as not installed. This
change fixes the issue.
Warn if prefixes don't map to site-packages directories.
ENH: Use a temporary directory for caching

If prefix/LOCAL-REPO is not writable, we use a temporary directory for
caching.
Add a warning for the user when the cache directory is not writable.
BUG: Print the correct prefix
Merge pull request #77 from enthought/prefix-support

Improve `--prefix` support