Allow completing from foreign namespaces (off by default).

This can be enabled by setting
omnisharp-auto-complete-want-importable-types to a non-nil value. This
currently only works for ido-mode and popup.el.

Completions from the server will have a tag appended to them (by the
server) like [using System.IO]. These kinds of completions insert the
completion like normally except they also add a using directive at the
very first line in the file. Point is not moved when the directive is
inserted, it's really nice! :)
Allow manually completing from all namespaces with the prefix argument

The completing from other namespaces, by default, the completing depends
on the `omnisharp-auto-complete-want-importable-types` setting. Calling
with a prefix argument (C-u) will now invert the setting.

This allows e.g. Normally completing from only the imported namespaces,
but in case nothing is found, the user can trigger a wider search.

It works the other way round too: if the user wants to seach all
namespaces by default, they can search for completions in the imported
namespaces only.