Fix issue #5. Add Type2D::XBar and Type2D::YBar

[DJDuque]

Add support for horizontal and vertical bar plots. This is done through the new Type2D::XBar and Type2D::YBar variants.

This PR breaks the old Type2D::Smooth(f64) and changes it to Type2D::Smooth { tension: f64 }. This keeps consistency with e.g. XBar and YBar, and should be the way to define new variants in the future. It is just much more clear to have the inner required data named rather than just a simple f64.

Something important to keep in mind (also noted in the documentation) is that bar_width and bar_shift in the Type2D::XBar and Type2D::YBar assume by default units of pt (Latex's pgfplots fault). So it is a priority to fix issue #12. This will make it much more easy to work with bar plots in units of the axis (with compat=1.7 or higher).

Usage of the new bar plots can be seen in examples/rectangular_integration.rs.