adding support for gdk
.gitingore: intellij stuff.
moved SdkManagerPlugin.[isOfflineBuild, hasAndroidPlugin] -> internal.Util.* + added javadocs for em
added groovydoc to SdkManagerPlugin.
added groovydoc to AndroidCommand.
added groovydoc to SdkResolver.
added groovydoc to Downloader & SdkDownload.
SdkResolverTest: fixed local.properties on Windows.
Moved implementation of SdkManagerPlugin -> SdkManagerAction (standalone).
Plugin now works for non-android projects, only resolves parts that don't require android closure. + Groovydoc
added com.jakewharton.sdkmanager plugin id alias.