Specify fields to run dev/build on

Hi guys,
No sure if this is something you want in core or not, but it's a handy feature to be able to exclude certain fields from a dev/build if you operate a really large database like I do. For example, we have a few large location tables that contain static location data that very rarely requires updating, so these tables can really slow down the dev/build process.

I've added the option to exclude certain tables, and vice versa, only run on specified tables via DEV_BUILD_EXCLUDE and DEV_BUILD_ONLY constants respectively. These can be defined in the ss_environment like so:

define('DEV_BUILD_EXCLUDE', 'GeoBlocks, GeoLocation, SiteTree');

I could also add some $_GET exclude and include queries if you think that would be of use to people.