Lookup and active system installed versions of gems

This patch adds functionality to lookup and active system installed (eg installed with yum/apt/rpm/deb) gems to bundler_ext. The specific versions of the gems to activate are using the native package system tooling, as made available through linux_admin. This allows bundler_ext to use/activate system packages while still allowing other versions of those gems to be available for use elsewhere.

This functionality is initially disabled w/ bundler_ext acting 100% the same as it did in the default scenario. If the end user sets the BUNDLER_EXT_SYSTEM env variable to true, the system dependencies will be looked up as described. To map Gemfile gem dependencies to system packages, the end user may also set the BUNDLER_EXT_PKG_PREFIX env variable, specifying a string to prepend to package names before looking up (for example rubygem-).

Specs have been updated / added to test new functionality.