Rollup of small fixes

- fixed bug in parsing of DEC strings (wcs.py)
- updated Task module to use with clauses instead of try/finally
Major update for scale (mapping) algorithms

The scaling algorithms (for mapping data values during rendering) have
been completely refactored.  They are now separated from the RGBMap class
and are pluggable.  Furthermore I have redone them modeled after the ds9
algorithms described here: http://ds9.si.edu/doc/ref/how.html

There are now seven algorithms available: linear, log, power, sqrt, squared,
asinh, sinh

The Preferences plugin has been updated to make the function a little
clearer, since there is some confusion also with the intensity map feature
that is part of the final color mapping process.