Bump leaflet to 0.7.3

Needs more testing
Categorized styles for remote WFS point layers #128

Such layers were not appearing. Now they are, and with categorized
symbol size, fill colour, and opacity pulled in from map.
Style categorized WFS line layers

Partial fix for #128
Style categorized WFS polygon layers

Part solution to #128
Merge pull request #130 from tomchadwin/master

Bump leaflet to 0.7.3/categorized styles for remote WFS layers (point, line, and poly)