Create a secure temporary directory for each file.

Rather than using a predictable filename in /tmp, use os.mkdtemp() to create a
secure temporary directory with a random name and create the file inside it.
This fixes a security issue: if another user on the system created a symlink
with a name likely to be used by rsub, it can allow files to be overwritten.

This also has several other advantages: the permissions on the directory prevent
other users on the system from reading the temporary file being edited, and
because a new directory is used for each file, it's now possible to edit
multiple files with the same name from the same client without them overwriting
each other.
Preserve the exact file size given by the client.

Rather than using strip() which deletes all trailing newlines in the file,
just truncate the received buffer to exactly the number of bytes specified by
the client, ensuring that files which originally did not end with a newline
continue not to have a newline, but files which ended with one or more still
have the same number. This fixes issue #11.