Removed topic rewriting.

Topics are no longer normalised from e.g a///topic to a/topic. This
matches the behaviour as clarified by the Oasis MQTT spec. This will
lead to unexpected behaviour if you were using topics of this form.
cit. @ralight from Mosquitto changelog.
Merge branch 'master' of github.com:mcollina/mosca
Refactoring and API improvements for #168.

Removed a function allocation for each message forward. This should
reduce the memory footprint in high-load applications.

@mocheng: please review this commit.