Load empty string as dimensionless.

@rhattersley in order to implement phase one of #1164 (and to be fully CF compliant) we need to interpret empty strings as dimensionless.  This PR implements this at the loading level.  However, `iris.unit.Unit("")` still translates to `unknown` as shown in the example below:

http://nbviewer.ipython.org/gist/ocefpaf/5fa5dc8b81a072389e61/staggered_grid_units_issue.ipynb