add plugins

$ git submodule add -f https://github.com/occ-corp/redmine_custom_field_aggregator plugins/redmine_custom_field_aggregator
$ git submodule add -f https://github.com/Tomohiro/redmine_text_format_selector plugins/redmine_text_format_selector
$ git submodule add -f https://github.com/occ-corp/redmine_gantt_view_overwrite plugins/redmine_gantt_view_overwrite
$ git submodule init
$ git submodule update
Merge pull request #4 from occ-corp/add_plugins