Bump version to 0.2.0 (the start of a new version)
Prevent from duplicating the selector form when tracker is changed.

The changed behavior of Redmine on rendering form attributes
caused the plugin view being duplicated when rerendering forms.

Change the view hook used to 'view_issues_new_top' from
'view_issues_form_detail_bottom', so that the plugin could be
renedered only once.

Github Issue: #1
Improve DOM structure: put inside the watchers_form

'watchers_select_multiple_by_group' is now span element,
and inside the 'watchers_form' element.
Add screenshot and modify README