Added the headerless release notes strategy.
Updated the release notes and the readme file with HeaderlessReleaseNotesStrategy.
Added GitHubReleaseNotesStrategy, refreshed release notes and the readme.
Added a test for the github strategy.
Simplified the readme.
Ported this plugin to use the GitHub release notes strategy.
Fixed sphinx release notes file cleaning.
Formatted release notes a bit.
Removed a max perm jvm option from the scripted configuration.
Small formatting changes.
Merge remote-tracking branch 'origin/feature/no-previous-releasenotes-file' into feature/no-previous-releasenotes-file

Conflicts:
	releaseNotesPlugin/src/main/scala/si/urbas/sbt/releasenotes/strategies/HeaderlessReleaseNotesStrategy.scala
	releaseNotesPlugin/src/main/scala/si/urbas/sbt/releasenotes/test/ReleaseNotesSelfTestPlugin.scala
	releaseNotesPlugin/src/sbt-test/blessing-produces-current-and-previous-release-notes/simple/test
	releaseNotesPlugin/src/sbt-test/headerless-release-notes/simple/project/BuildConfiguration.scala
	releaseNotesPlugin/src/sbt-test/headerless-release-notes/simple/test