Add getUrl methods to TileWMS and ImageWMS

As [discussed](https://groups.google.com/d/msg/ol3-dev/dYPNLdUjuXM/RZgCmWfSIbMJ) on the mailing list this PR adds `ol.source.TileWMS#getUrls` and `ol.source.ImageWMS#getUrl` methods.

Please review.