SI-6766 Makes the -Pcontinuations:enable flag a project specific preference

Make the continuations plugin flag specific to the continuations library.
Remove the documentation about adding the continuation plugin from
the eclipse README.md.

One consequence of this change is that if a programmer has prefs
for the continuations eclipse project then there will be a merge conflict.
That's unavoidable without having a more sophisticated way  to build
the eclipse projects.
SI-6766 Create a continuations project in eclipse

We're missing a continuations dependency when we try to build reflect
in eclipse which causes things to die horribly. This commit adds an
eclipse project for the library portion of continuations, makes
projects that depend on the scala-library also depend on
continuations-library, and adds another bullet to the eclipse readme to
turn on the continuations plugin.
Merge pull request #1867 from huitseeker/backport/1721

SI-6766 Create a continuations project in eclipse ( backports https://github.com/scala/scala/pull/1721)