Add a Graticule component

This PR adds a graticule component to the library.

The graticule component uses the immediate API to draw meridian and parallel lines on the map.

The creation of the meridian and parallel line strings uses an adaptive subdivision algorithm (the code is based on code from @twpayne in #1884).

The graticule component works with the Spherical Mercator projection, as well as other projections. The patch includes two examples, [one](http://erilem.net/ol3/graticule/examples/graticule.html) that use the Spherical Mercator projection, and [one](http://erilem.net/ol3/graticule/examples/sphere-mollweide.html) that uses the [Sphere Mollweide](http://en.wikipedia.org/wiki/Mollweide_projection) projection.

The patch also adds maxLat, maxLon, minLat, and minLon properties/options to ol.proj.Projection, which are required values for the graticule component. I'd like to make sure that adding these properties makes sense to others.

Also, the graticule component doesn't draw labels at this point. I have a [graticule2 branch](https://github.com/elemoine/ol3/blob/graticule2) whose [graticule example](https://github.com/elemoine/ol3/blob/graticule2/examples/graticule.js) includes code that adds labels at the intersections between the meridian/parallel lines and the map viewport. I removed that code from this PR's branch, to make the example simpler. My graticule2 branch also includes [code](https://github.com/elemoine/ol3/blob/graticule2/src/ol/geom/flat/intersectionflatgeom.js) for calculating the intersection between a line string and a segment. This is used in the example for the label placement.

Please review.
Still interested in finding time to look into it.  I guess it may depend on the definition of a validity extent.  I have in my head that it is an extent for which `distance(forward(inverse(p)), p) < tolerance` for all points `p` that intersect the extent (assuming `forward` and `inverse` are transforms from and to geographic, `distance` calculates distances, and `tolerance` is some small number).  Not sure if this is what others consider it (or if it actually holds).