Added new unit definitions for hPa and mb.
Merge pull request #30 from rsenk330/mb-hPa-units

Added new unit definitions for hPa and mb
supports numpy.fabs, fixes #51
enable subclassing of Quantity

see issue #56 for detailed discussion
fixes squeeze bug introduced in recent numpy release
Merge pull request #57 from lumbric/patch-1

enable subclassing of Quantity