Add geodesic option for measure

[bartvde]

Add geodesic measurement option to the measure example.

[bartvde]

Okay I've cleaned this up @ahocevar according to our discussions. To summarize:

• use harversine distance on the sphere for distance measurements
• add vincentyArea function (and pass on the coordinate array of the linear ring) on the sphere, include the same reference as in OpenLayers 2 since the code is a "migration" of that code
• do not export ol.sphere.WGS84 but use the same snippet as is already in the API docs for ol.Sphere (so specify the radius in the example)

[ahocevar]

Minor, but if you assign coordinates.length to a variable before assigning x1, you can reuse it instead of dereferencing coordinates.length 3 times.

[bartvde]

@ahocevar I incorporated your latest review comments, let me know if you have more coming

[ahocevar]

Looks good now. Please merge when Travis is happy.

[bartvde]

thanks for the extensive review and discussions @ahocevar !