Export ol.Sphere#circle function
Add Tissot indicatrix example
Don't use a named color for defaultFillStyle

Named color support is optional (see ol.color.ENABLE_NAMED_COLORS)
Use ol.color.ENABLE_NAMED_COLORS in tests
Merge pull request #1984 from fredj/export-circle

Export ol.Sphere.WGS84.circle function
Merge pull request #1985 from fredj/named-color

Handle ol.color.ENABLE_NAMED_COLORS = false
Use ol.extent.getWidth and ol.extent.getHeight