aselect: New aselect-module.

New A-Select module with an updated A-Select authentication source. The
new module gives us support for request signing, but we lose support
for A-Select Cross.

Thanks to Wessel Dankers for creating this module, and to Thijs
Kinkhorst for providing the patch.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@3091 44740490-163a-0410-bde0-09ae8108e29a