PERSONDIR-61: handle multi-valued attributes from Shibboleth SP

dixit https://wiki.shibboleth.net/confluence/display/SHIB2/NativeSPAttributeAccess:

	  multiple attribute values are separated by a semicolon,
	  and semicolons in values are escaped with a backslash.
Merge pull request #3 from prigaux/PERSONDIR-61

PERSONDIR-61: handle multi-valued attributes from Shibboleth SP