Refactor frametools.plotframe to not read from disk.

Allow plotting from memory by introducing a lower level
function plot_frame that doesn't try to read from disk.
Merge pull request #90 from ketch/plot_from_memory

Add frametools.plot_frame that plots but does not read from disk.
Add option plotdata.plotaxes = image for scaled, tight axes.
Merge pull request #91 from ketch/axis_image

Add option plotdata.plotaxes.image for scaled, tight axes.