Made the VizieR server name an astropy configuration item.
Made the SIMBAD mirror a ConfigurationItem as well.
Moved VIZIER_SERVER ConfigurationItem to root of package.
Merge pull request #101 from jwoillez/vizier_config_item

Make the VizieR & SIMBAD server names astropy configuration items.
API change for discussion today
API - astropy-based example
API cont'd
API updated from June 7 discussion
API examples updated again
added edit_on_github extension
API update - include some specific result caching
typo fix - no 'else' in write
API notes cont'd