Parse gdal xml file

[ri0t]

gdal2tiles creates a xml metadata file along each tilestache describing things like

• extents
• origin
• projection
• srs
• tilef ormat data

These should be available as optional metadata for layer objects

[ri0t]

projection, srs and tileformat data are always the same, so i omitted that, as well as origin (which is just the first extent-corner)