Add a new axvline function that allow to draw vertical lines

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I've added a new axvline function that allow to plot a vertical line at a given 
x location. This is similar to matplotlib's axvline :
http://matplotlib.org/api/pyplot_api.html#matplotlib.pyplot.axvline

The change involves adding a new VLine class and an axvline function.

I've also modified the sample application to draw two green vertical lines.

Here is the commit :
https://github.com/julienr/cvplot/commit/d3f7a22292cba0db877ceae68c3be1e82d0fa8a
3

You can pull from my axvline branch :
https://github.com/julienr/cvplot/commits/axvline

Original issue reported on code.google.com by julien.r...@gmail.com on 23 Apr 2013 at 11:09