Add EventCollection and eventplot

As discussed in the mailing list, here is the event plot. It's purpose it to plot one or more series of 1D data points along horizontal or vertical lines. It is dived into two parts.

The first part is the EventCollection collections class, which is a subclass of LineCollection that makes it easy to create and manipulate individual rows of vertical lines or columns of horizontal lines. Besides being used in the final plot type, this is also useful on its own, for example for marking the locations of data points along an axis (see eventcollection_demo.py). Besides the convenience functions, since it is a subclass of LineCollection it also exposes all internal LineCollection and Collection methods, allowing more fine-grained modifications for artists.

The second part is an axes method (also in pyplot through boilerplate.py) which is given one or more rows of data points and creates corresponding rows or columns of lines. All rows or columns can be given the same line properties or individual line properties (see eventplot_demo.py). It returns a list of EventCollection objects.

Tests for both EventCollection and eventplot are present, and the files have been tested for pep8 compliance and run through pyflakes and pylint and fixed wherever appropriate.