Feature plot_iq

Extends `pydarn/plotting` by adding the ability to plot iqdat files. This plotting routine takes a `beamData` object from `pydarn.sdio.radDataTypes` as input and plots the `mainData` I and Q channels. Here are the function's options:

1) sequences: (list of ints or None) Defines which sequences of voltage data to plot. Default is None which plots all.
2) mag_phase: (boolean) Specifies whether magnitude and phase should be plotted instead of real and imaginary.
3) scale: (None or float) Specifies the scaling to use on real, imaginary, or magnitude axes. Default is None which auto-scales.
4) user_ax: a matplotlib axis object
5) tx_pulse: (boolean) Specifies whether or not to plot Tx pulses
6) int_data: (boolean) Specifies whether or not to plot voltage samples from the interferometer array (checks beamData.prm.xcf)

To test the plotting routine, please run: `python iqPlot.py` and read through the output to see if the plots are what they should be. 

Here is an example usage:

    myPtr = pydarn.sdio.radDataOpen(datetime(2012,5,21),'kap',fileType='iqdat')
    myBeam = myPtr.readRec()
    pydarn.plotting.iqPlot.plot_iq(myBeam)

The `__init__.py` file in the `pydarn/plotting` directory was modified to add an import all from iqPlot.py.