New filter: core:ScopeFromAttribute

Retrieve a scope from a source attribute and add it as a virtual target
attribute.

Filter created by Thomas Zangerl <thomas.zangerl@freenet.de>

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2218 44740490-163a-0410-bde0-09ae8108e29a