capture dct:spatial as bbox, if it contains a box

[pvgenuchten]

in some cases, bbox is strored in dct:spatial, nice to have it as bbox

added some csw parse tests, seemed not existing

[coveralls]

coverage: 60.296% (+1.3%) from 58.984%
when pulling e81b2e5 on pvgenuchten:if_dct_spatial_has_a_box
into 1354d06 on geopython:master.

[geographika]

@pvgenuchten - great to get some tests for the CSW parsing. It looks like there is a lot of shared code in the __init__ for CswRecord and Csw30Record that could be refactored into one function - maybe for another pull request.

A couple of questions:

1. If a document has both ows:BoundingBox and dct:spatial then self.bbox will overwrite the value from ows:BoundingBox with dct:spatial. Is this scenario possible, and if yes then should this be the correct precedence?
2. If dct:spatial is not a bbox in the form -180,-90,180,90 then self.spatial is set to val. What could val be - a string? It would be good to have an example document and test for this.