Allow programmers to specify build path manually

Currently the build dir for mruby is always `#{MRUBY_ROOT}/build/#{@name}`, this commit allows users to customize build dir using the following syntax in `bulid_config.rb`:

``` Ruby
conf.build_dir = "/tmp/mruby-build"
```

The mrbgems checked out using `git` and the generated files will all be redirected to this folder.