Drop sqlite - Persistent cookies with file IO

Hey @oberstet, this is work towards #64.

Sorry this took so long, I got caught up with work. I'll have this weekend and Monday to work on this issue.

Some notes:

1. The way `setAuth` works is inefficient, it ends up rewriting the entire file in order to perform the update. Do you know of a better way to do this?
2. We don't have to check for `HAS_ADBAPI` so `PersistentCookieStore` will always be available during runtime.
3. `if id in self._cookies:` in `setAuth` is redundant since it is checked on the parent method. Should we return a boolean from it to avoid the second check?
4. On `setAuth` I have to call flush to make sure all the cookies are available. Should I call flush on `create`? 
5. Is there a better way to do `IO` in twisted, since all of this is blocking.

Let me know where I can improve these changes, like I said I'll have some time available to work on this!