Allow 1D plots to have the coordinate on the y-axis.

This PR adds an optional argument to the `iris.plot.plot` and `iris.quickplot.plot` functions that allows the coordinate to be presented on the y-axis of a 1D plot (see #581). This is necessary in order to make profile plots with iris, these types of plots are extremely common in meteorology and oceanography (temperature/salinity/velocity profiles in the ocean, temperature profiles in the atmosphere to name a few examples).

##### Implementation notes:

This change is implemented as a new boolean keyword argument. I am aware that this is not to many people's taste, but having explored other options I felt this was the best way to go for the following reasons:
* Guessing of vertical coordinates is quite difficult to do well. If we tried to automatically transpose the axes for plots of a cube with a vertical coordinate we are likely to miss some cases, and worse, misidentify others. I think this might end up disappointing the users.
* `iris.quickplot` needs to know how to draw the plot labels, so the information needs to be available at that level.
* Having a keyword argument to `_draw_1d_from_points` makes life easier for other modifications (e.g., #522) since they can easily tell if the axes will be transposed.

I'd appreciate some feedback on this. If anyone has a better way of doing this that remains flexible I'd be happy to hear it.