Refactor FileSystemResolver

[jpstroop]

• Make the cache a class (implement __get__, get, and __set__ to hide file copying and deleting, and internally manage a SafeLruDict)
• Make a ResolvedFile class (or something like that), that can hold a file's path, cache path, last mod, tell if it's stale, etc. See note in loris/resolvers/file_system_resolver.py

[jpstroop]

Alternatively, if we didn't want to go this far (it's not as clean as the above makes it sound) then move the cache behaviors into a superclass or Mixin so that other cache implementations can use it.

[jpstroop]

Also #145