Add support for installing gems into projects that use Bundler

[bguthrie]

In order for build-via-Rake-command to make any sense at all, a modern CI server must check for the existence of a Gemfile and honor it if necessary by resolving dependencies to Bundler's satisfaction.

This is currently working in a limited sense and CC.rb will honor Gemfiles if they exist by bundle gems into the vendor path that's local to the project. But the bundle install command needs to be properly configurable in order to be useful, and the execution of it needs to be simpler (todo: switch to ruby -S bundle ... rather than trying to resolve the path to the Bundler binary by hand.)

[bguthrie]

A serviceable implementation of Bundler integration is in place and I'd prefer to track future problems with this feature in finer-grained pieces. Closing this one out.