Merged hooke.plugin.plotmanip into hooke.plugin.curve.

Also:
  * Fixed a number of typos in hooke.plugin.curve.
  * Rewrote derivative code for Numpy in
    hooke.util.calculus.derivative.
  * Removed FFT code (I'm replacing it with my FFT_tools module
    shortly).
  * Moved v-clamp-specific code into generalvclamp.
  * Moved plotmanips to hooke.plugin.plotmanip holding area.