Remove plt usage that I accidentally added.
Merge pull request #46 from ketch/fix_plt_bug

Remove plt usage that I accidentally added.
Interactive plotting without a plotdata object: iplot.py.
Remove unused imports, fix small bugs.
Abstract away most of the functionality of Iplotclaw into Iplot.

The Iplot class does not need to use Clawplotdata objects
and is generally ignorant of anything Clawpack-ish.  It simply
provides an out layer for looping through plots, along with
simple caching of frames.  It could conceivably be used in a
wide range of contexts.