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).
Testsuite enhancements from OpenLayers/GeoExt 1

See openlayers/openlayers@57c16b7 & geoext/geoext@aaab5eed.
Enable headless testing through casperjs.

This enables us to test 1010 of a total of 1062 single tests without opening the
test suite in a browser. We skip some tests as we currently are not able to mock-up
certain calls of the ancient Test.AnotherWay suite. The total running time is usually
reduced by about 50%.

Call the script from the root of a GeoExt checkout as follows:

  ./tests/headless/casperjs-1.1-beta.1/bin/casperjs test tests/headless/run-testsuite.js

This also marks the first important step towards continous integration tests, e.g. through
Travis.
Do not depend on `parent` being present.
Destroy leftover mappanels prior to guessing.
Swapping undefined/null when conmpairing.

This is actually more precise than what we had before.