stabilize DecWriter and make writers closable

This commit does two things:
 - It makes the `DecWriter` the "inverse" type
   for `EncWriter`. This includes handling panic'ing
   of the inner writer, adding documentation, runnable
   examples a.s.o.
 - It introduces the `Close` trait which is implemented by
   `EncWriter` and `DecWriter`. Both should be closed
   explicatly after processing data to complete the
   en/decryption process and handle potential errors. (e.g. decryption fails)

Now, the `flush` implementation of `EncWriter` and `DecWriter`
only flushes the inner writer. An `EncWriter` or `DecWriter`
cannot really be flushed since flushing the buffered data effectively
means "creating" the final fragment such that no more data can be
written.
By only flushing the inner writer, `flush` can now be called multiple
times and still cause e.g. writing any buffered (by the inner writer)
out. If calling code want to complete the en/decryption it should call
`close`.

Author: Andreas Auernhammer