Missing I18n

The strings in the plugin were not wrapped in i18n functions, this pull request fixes it. The text domain is loaded in a way that should work regardless of whether CMB is being loaded inside a plugin/theme or as a mu-plugin. Language files are looked for in the following locations

/wp-content/languages/custom-meta-boxes/
CMB_PATH/languages/

The loading of textdomains inspired by http://geertdedeckere.be/article/loading-wordpress-language-files-the-right-way

Finnish (locale 'fi') strings added for testing purposes, feel free to remove them.