Store & check whether the map is already rendered.

In case we have once been rendered, we can check the newly introduced
mapRendered-property instead of compairing DOM-elements in the
onResize-method.

This also fixes a reported unexpected behaviour when one was manually
rerendering a mapanel's OpenLayers.Map-instance to another element (#209).
Merge pull request #209 from marcjansen/rendered-boolean

bug: Map Panel always reattaches the map to the panel