plugin settings for default code language

This branch adds a codebutton settings page in _Administration_ > _Plugins_ to configure the default language in the dropdown menu.

This is inspired by @mindo's changes... :)

Feel free to merge the changes! I also bumped the version number to 0.1.0

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And by the way: You probably have to rename the plugin in the redmine.org repo because you now have two different plugin identifier!

Your plugin is registered as "redmine_codebutton" in each redmine instance ( ``Redmine::Plugin.register :redmine_codebutton do` ) but the redmine plugin update check (introduced in 2.5.0) can't find the plugin in the redmine repo because there it is named "codehightlight_button" (with a typo, btw :smirk: ).