Implement gembox-relative gemdir paths

In the case where a relative path is specified to a gembox from
build_config.rb, it was previously tricky to specify relative gem paths
from inside that gembox.

For example, consider a project in which mruby is checked out as a
submodule in the project root:

    +- project_root
       |
       +- mruby/
       |  |
       |  +- build_config.rb
       |  |
       |  +- ...
       |
       +- my_gembox/
          |
          +- my_gembox.gembox
          |
          +- my_gem/
             |
             +- mrbgem.rake
             |
             +- ...

If build_config.rb refers to my_gembox with a relative path, it's
difficult for my_gembox to then refer to my_gem. With this proposed
change, my_gembox.gembox can look like this:

    MRuby::GemBox.new do |conf|
      conf.gem "my_gem"
    end

@bovi, @matz