Core Extensions relocation

Following up on the idea we've been exploring for making some extensions 
more prominent, attached is the first rev that I think may be worth 
considering seriously.  Main improvement from the last is that I 
reorganized the docs to break out what I decided to tentatively name 
"Core Extensions" into their own chapter.  No longer mixed in with the 
rest of the contrib modules, and I introduce them a bit differently.   
If you want to take a quick look at the new page, I copied it to 
http://www.2ndquadrant.us/docs/html/extensions.html

I'm not completely happy on the wordering there yet.  The use of both 
"modules" and "extensions" is probably worth eliminating, and maybe that 
continues on to doing that against the language I swiped from the 
contrib intro too.  There's also a lot of shared text at the end there, 
common wording from that and the contrib page about how to install and 
migrate these extensions.  Not sure how to refactor it out into another 
section cleanly though.

Regression tests came up last time I posted this.  Doesn't look like 
there are any for the modules I'm suggesting should be promoted.  Only 
code issue I noticed during another self-review here is that I didn't 
rename contrib/pgrowlocks/pgrowlocks--unpackaged--1.0.sql cleanly, may 
need to do that one over again to get the commits as clean as possible.

Updated code is at 
https://github.com/greg2ndQuadrant/postgres/tree/move-contrib too, and 
since this is painful as a patch the compare view at 
https://github.com/greg2ndQuadrant/postgres/compare/master...move-contrib 
will be easier for browsing the code changes.

-- 
Greg Smith   2ndQuadrant US    greg@2ndQuadrant.com   Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support  www.2ndQuadrant.us