Add option to prevent load of excluded groups

Hi,
even if I know that in bundler FAQ ability to require all spec for all groups is mentioned as feature it is not good behavior in some use cases.
One of them is our use case:
We develop virtual machine with requirement on minimal size dependency. We found bundler useful to handle multiple gem version in case, that there is more rails servers ( our server is just configuration server on specified port ). We ensure that version is correct different way, so we use bundler only to preload correct gem versions. We have quite a lot of gems in development and test groups, so for us is not option to have such gems on target system. Also we need to handle systems which is offline to public net, so downloading it is not option.

So what I change?
I add option miniresolver which solves only non-excluded groups. So if someone want on production miniresolving then creating .bundle/config which contain without and miniresolver, then it prevent loading of specs and resolve on required groups.

Thanks for attention
Josef

P.S. it would be nice if you can backport it also to 1.0 as we still use it ( change is same, just maybe testcase doesn't work, I can create separate sr if you are interested ).