Configuration: added getString and getBoolean functions.

This commit adds two convenience functions: getString and getBoolean to the
configuration class. These functions will performe type checking before
returning the value, and has support for required and optional values.

Also update getValue to support required options.


git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@630 44740490-163a-0410-bde0-09ae8108e29a