Add scale/offset filter support.

This pull request adds support for the HDF5 scale/offset filter (http://www.hdfgroup.org/HDF5/doc/UG/10_Datasets.html#Scaleoffset) for (usually) lossy compression of integer and floating-point data. (Previously submitted as a patch by e-mail to Andrew Collette 2012/11/16, reproduced here for possible convenience.) Partially addresses issue #61.