Add compiler plugin continuation to project settings
(cherry picked from commit 171aab711a648e944318f598d805553f8c33ebb6)
Changed implementation acording to Mikko Peltonen guides in the comment on the pull request.
(https://github.com/mpeltonen/sbt-idea/pull/203#issuecomment-13190607)

Extracted the scalacOptions mapping to a map and added translation of "-explanintypes", "-optimise" and "-nowarn".

The defined mappings will no longer be inclueded in the compiler options tag as idea don't do this.

All paths with -Xplugin: will be include in the plugin path list
(cherry picked from commit 45d6a26cec8edaee4fcde3f60694c1ac49d83c6c)

Conflicts:
	src/main/scala/org/sbtidea/SbtIdeaPlugin.scala
Use the xplugin val to generate pluginPaths thanks to Eugene Platonov for pointing that out.
(cherry picked from commit 19d8f7ebe0016ea0661428d4ea4a824f0fae90e5)
Add test for #203(cherry picked from commit 67cdfac04b9dafac5f471ab9ccb500a7fd030a84)
Minor code cleanups(cherry picked from commit d8502291645e8ade77f866541421b15e508fe923)
Fix test
Evaluate 'managed-sources' task if project has source generators

This is needed to have all generated source folders created and available in
Idea metadata.
(cherry picked from commit 41b7302987c00effc94c0cd8c5027a9067d6c1f6)

Conflicts:
	src/main/scala/org/sbtidea/SbtIdeaPlugin.scala
More optimistic log message(cherry picked from commit 988e6fb80d8ee7a5312f41f087c84eca3f12e598)

Conflicts:
	src/main/scala/org/sbtidea/SbtIdeaPlugin.scala
Use relative instead of absolute paths(cherry picked from commit 364fc97e2725577accd6e2953c6d951e943fb692)
Fix path relativization issue(cherry picked from commit 2c2d7c59ab56bf93e84da0b43ce10f6258b245e0)