[tidy] Remove blank line
[build][tidy] Few tweak in libxively makefile -

* move around lines to reduce duplication
* the mqtt codec is easier to compile from subdir
[build][import] Move mqtt codec to subdir -

This is way easier for now to avoid messing with the build system.
Once we clarify the licence of mqtt codec, we can see whether it needs
to leave under import dircotry.
[build][fix] Add missing sources for mqtt layer
[build] Fatten the makefile even more
Merge pull request #98 from xively/makefile_cleanup

Makefile cleanup