add getResolutionFromZoom() method to Map, Layer

[euzuro]

Originally from tschaub's patch from #228:

Should we add a getResolutionFromZoom() method which allows user to specify a zoom level and get the corresponding resolution?

[euzuro]

my original thoughts (again from #228):

I would like to offer this functionality, but at least right now, we cannot put that fix in, because not all baselayers can support this functionality. baselayers whose resolutions arrays we calculate can of course do this and without problem, but the whole idea behind the FixedZoomLevels? functionality in use by 3rd party baselayers like google, ve, etc is that we *dont*know* the resolutions array and rather try to work around it.

[euzuro]

from chat with cr5:

I think that it may make sense to implement the function (although not for 2.2) and return a resolution for the zoom if we have it, and otherwise define what we're going to return: false, or null, or what have you

[crschmidt]

I've thought about this some more -- isn't getResolutionFromZoom equivilant to:

layer.resolutions[0]?

If so, do we really need a wrapper around this?