Fix http://trac.osgeo.org/openlayers/ticket/3554

Finally using Util.isElement and nothing else for cross-platform
compatibility according to elemoine's comments.

Tested on Chromium 18 Ubuntu 12.04.
8 tests fail (detailed: fail 9). List follows.

23:13:26 test_respectZoom: Google tests can't be run from localhost:8000
23:14:22 test_write_wmc_Layer: Safari has wierd behavior with getElementsByTagNameNS: the result is that we can't run these tests there. Patches welcome.
23:16:15 test_Layer_Google_constructor: Google tests can't be run from localhost:8000
23:16:15 test_clone: Google tests can't be run from localhost:8000
23:16:15 test_Layer_Google_isBaseLayer: Google tests can't be run from localhost:8000
23:16:15 test_Layer_Google_Translation_lonlat: Google tests can't be run from localhost:8000
23:16:15 test_Layer_Google_Translation_pixel: Google tests can't be run from localhost:8000
23:16:15 test_Layer_destroy: Google tests can't be run from localhost:8000
23:16:15 test_Layer_Goole_forwardMercator: Google tests can't be run from localhost:8000
23:16:15 test_Layer_Google_overlay: Google tests can't be run from localhost:8000
23:16:15 test_Layer_Google_isBaseLayer: Google tests can't be run from localhost:8000
23:16:15 test_Layer_Google_setGMapVisibility: Google tests can't be run from localhost:8000
23:20:13 test_Layer_MapServer_Reproject: Can't test google reprojection in Opera or Safari.
23:20:18 test_Layer_WMS_Reproject: Can't test google reprojection in Opera or Safari.
Merge pull request #446 from candeira/fix_ticket_3554

Fix for trac ticket # 3554 single-arg Map constructor does not work if you pass a DOM element ref