Fix typo in configuration.rst
Add to docs about mapfile requrements

The mapfile used must support WMS requests or you'll get back a bunch of ``msWMSDispatch(): WMS server error. WMS request not enabled. Check wms/ows_enable_request settings.`` errors
Merge pull request #66 from pnorman/patch-2

Fix typo in configuration.rst
Merge pull request #67 from pnorman/patch-3

Add to docs about mapfile requrements
Merge branch 'master' of github.com:mapproxy/mapproxy
raise ConfigurationError for unknonw grid base