Adding XSD to basemaps and tile types

[MattReimer]

This change adds support for the following layer types in the XML. It relates to #74

After this change you'll need to add type="wms|tms|xyz" tag to the layers. For now wms, tms, xyz are the choices. I'm open to extending this list.

Also the Basemaps.xml file is hooked into an XSD so watch it for red carefully when you're editing it.

<Layer name="Watershed Boundary Dataset" type="wms" url="https://hydro.nationalmap.gov:443/arcgis/services/wbd/MapServer/WmsServer?">

NOTES:

• This change allows you to specify new baselayers. it does NOT implement that functionality in ArcRave or QRave.
• Test this against QRave before merging. It's a pretty safe fix and it shouldn't break anything but I've definitely said that before. UPDATE: Quick test seems to be backward-compatible with QRAVE 0.4.0
• I'll need validators for this but I've got it covered here Create a webrave symbology validator #77