Plotlabel

Added a "plotlabel" argument to the PlotItem. The plot label is like a title, but it goes in the upper left corner of the plot (in the layout position (0,0), which was an unused layout position in the PlotItem). The purpose of the label is to be a panel identifier in a figure (panel A, B, C, D). The label is a LabelItem, so that it can be formatted with HTML, etc. 

Also in this pull is "Legend_Updates" (merged earlier), which allows the line shown in the legend to be horizontal rather than diagonal. This may be preferable when a large number of items are present in the legend, and the horizontal orientation is the standard/default for most graphics programs.

A merge with the current develop branch also appears here - it should have no effect since only PlotItem and LegendItem actually have changes here. The merge was done earlier.