This removes the currently unused "limit" parameter from
client:login_with_password() and client:register_with_password(), and adds a
new "no_sync" parameter. When the flag is set, the call to client:_sync() is
skipped. This can be useful for small scripts which do not need to do a sync
after login, yet they would rather user the higher level "matrix.client"
instead of the low level "matrix.api". For example, a simple script to send
a message to an already-known room given its ID could skip the sync in order
to be faster, without needing to revert to using "matrix.api".
