GemBox

This is based on the discussion in #959

I would like to introduce a new GEM feature which is called GemBox (name suggested by @masuidrive). A GemBox is nothing more than a specification which contains a list of GEMs. Such a list is full-filling a specific purpose (i.e. make mruby iso compatible). Instead of including every single necessary GEM with ```conf.gem 'path/to/gem'```, I implemented a new method called ```conf.gembox 'path/to/gembox'```. Such a GemBox file could look like this:

```
MRuby::GemBox.new do |conf|
  conf.gem "#{root}/mrbgems/mruby-sprintf"
  conf.gem "#{root}/mrbgems/mruby-print"
end
```

For your reference I created two example GemBoxes:

* full-core (contains all GEMs provided in the mruby source)
* default (contains all GEMs we build by default into a mruby binary)

I modified ```build_config.rb``` to use the default.gembox and ```travis_config.rb``` I modified to use the full-core.gembox.