Add hooke.plugin.curve (moved from hooke.plugin.curvetools)

Add hooke.plugin.curve (moved from hooke.plugin.curvetools)

Provides 'curve info' and 'export' commands.  Added test.curve_info
to test the 'curve info' command, but it currently fails due to
unimplemnted Curve loading.

Also:
 * Minor docstring cleanups in hooke.plugin.playlist
 * Moved some functions to hooke.plugin.generalvclamp.  I'm not clear
   on Experiment-specific Curve interpretation yet, but these
   functions (which will eventually be Commands) seem better placed
   here.
   * fit_interval_nm
   * find_current_peak
   * pickup_contact_point
   * baseline_points