Simpler configuration check allows not-installed to be caught earlier.

Issue #138
Abstracted out loading of openjp2, openjpeg libraries.  #138

That much was common to both load_openjp2 and load_openjpeg.  Reduces
number of different moving parts.
More refactoring, fewer branches.  #138
Prepping 0.5.7 release.
Merge branch 'issue138'