Implement cache management strategy

[MobyGamer]

Currently there is no cache management strategy: Nothing is cleaned up, and if there is no space for a title, it will fail during extraction phase. We should have a cache management strategy for when the cache doesn't have enough space to decompress a title.

Move all cache management into unit. Then:

• Come up with cache purge strategy (multiple strategies in the future, but for now can just trash the first one, then next one, etc. until enough space is available)
• "do we have space for this" test can be limited to 5 dirs for testing and then use avail disk space for final code
• "how much space do we need" test should determine exact amount of storage needed from archive header, but can estimate 4*archive size for testing