Implementation of :core parameter for conf.gem

When we currently look at the build configuration we usually see something like this:

```
# Use standard Kernel#sprintf method
conf.gem "#{root}/mrbgems/mruby-sprintf"

# Use standard print/puts/p
conf.gem "#{root}/mrbgems/mruby-print"

...
```

I don't like this for two reasons:

1. we are using a magic variable *root* which points to the mruby root.
2. we hardcode the directory structure of mruby into a configuration

With this patch I propose we use a syntax for core GEMs like this:

```
# Use standard Kernel#sprintf method
conf.gem :core => "mruby-sprintf"

# Use standard print/puts/p
conf.gem :core => "mruby-print"

...
```

I think using core GEMs should be as easy as possible. And at the moment it seems easier to configure a GEM via github than inside of the core. I hope this change will change this.