Adding missing "centi", "deci", "deca" and "hecto" SI prefixes

Hello,
I was surprised that four SI prefixes were missing, so I suggest to add them if there is no good reason to let them appart.
I did not fully test the changes but it should be fine like this. I don't think any other changes are required, except for the doc which I can edit later if you accept the pull request (unless you generate the "allowed as attributes and arguments" lists automatically).

regards,
Luc

edit : reference for SI metric prefixes -> http://en.wikipedia.org/wiki/Metric_prefix