Improve documentation of Plot methods

[Azaya89]

closes #1536

This PR adds API reference examples to the following plot methods:

• [x] bar @Azaya89
• [x] barh @Azaya89
• [x] scatter @Azaya89
• [ ] line
• [x] hist @Azaya89
• [x] heatmap @Azaya89
• [x] box @Azaya89
• [ ] scatter_matrix
• [x] labels @Azaya89
• [x] kde/density @Azaya89
• [x] area @Azaya89
• [x] table @Azaya89
• [ ] andrews_curves
• [ ] points
• [x] ohlc @Azaya89
• [ ] image
• [x] violin @Azaya89
• [ ] hexbin @Azaya89
• [x] quadmesh @Azaya89
• [ ] lag_plot
• [ ] polygons
• [x] step @Azaya89
• [x] bivariate @Azaya89
• [ ] parallel_coordinates
• [ ] errorbars
• [x] contour @Azaya89
• [x] contourf @Azaya89
• [ ] vectorfield
• [x] rgb @Azaya89
• [x] paths @Azaya89
• [x] dataset @Azaya89
• [ ] explorer

[Azaya89]

The signature defs in the scatter plot method still needs to be updated I think, as well as adding some more examples to it.

[Azaya89]

I just pushed 8878346 to reset this PR so i can figure out the directory tree much more easily. Still a WIP.