change missing_indexes_generate_script to use spgist if available

[robe]

At a cursory glance it seems spgist on character fields performs better than btree for LIKE searches and LIKE seems to work fine with it. Still need to do some benchmarking, but since we have a script to generate the indexes, it is easy to swap out btree in favor of spgist if a user is running 9.2+ and reap the benefits without making any of the geocoder query logic 9.2 specific

This just required a change to to check the version of PostgreSQL and use spgist if available ​http://www.postgis.org/documentation/manual-svn/Missing_Indexes_Generate_Script.html

[robe]

we won't be supporting spgist in 2.1.0 so this by dependency needs punting

[robe]

Milestone renamed