Moved package from exporters to export

But still doesn't work yet -- this commit just includes the move of the
package--links still need to be fixed.
Changed names of classes from Export* to *Exporter

Changed name of base class from Export to Exporter as well.
Changed comments, docstrings, etc.
Refactored pysb.export and docs

Added a __main__.py module to pysb.export so that pysb.export can be run
as a script.
Added docstring as <notes> in sbml export