~ Update README about #213
[ci skip]
* Rename I18n::JS.export_dir to .default_export_dir_path
* Rename I18n::JS.config_file to .config_file_path
* Make config file path configurable
* Copies i18n.js to customised path on rake i18n:js:export
* Move I18n::JS.default_export_dir_path to I18n::JS::DEFAULT_EXPORT_DIR_PATH
~ Update README about exporting
! Fix spec for MRI 1.9.3
~ Update README about #213
[ci skip]