support tile extent for grid layers

[tschaub]

Layers that extend Grid base their tile lattice on the layer's maxExtent property. In practice, the maxExtent of a layer corresponds to the data bounds or the map bounds imposed by the user. Neither of these is necessarily coupled to the tile extent of a tile caching service.

The  WMS-C recommendation and other standards have the server advertise a tile extent. It makes sense for OpenLayers to support a tileExtent property for gridded layers.

[tschaub]

Tests and example to follow.

[tschaub]

Now with example and tests.

[tschaub]

support tileExtent

[ahocevar]

A clean solution with very few lines of code, saving users a lot of application specific code. Great work. Assuming that tests pass, please commit.

[tschaub]

In w/ r11002