Provide cache invalidation

(sx-cache--invalidate): Invalidate provided cache.  Allows for
invalidating variables associated with cache using `makunbound`.  Cache
can be reinitialized using arg `init-method`.
(sx-cache-invalidate-all): Invalidate all caches then call
`sx-initialize` to reinitialize.  Arg `save-auth` prevents access_token
from being lost.
Fix docstring.
Use `cl-remove-if` rather than `remove-if`
Merge pull request #73 from jleechpe/invalidate-cache

Provide cache invalidation