Add permission syntax to allow class/method/field

WHAT
The class permissions can only deny access to methods/fields. It is cumbersome to use when only a few methods need to be allowed. Adding a syntax to explicitly allow class members would also enable easier composition from the RESTRICTED permissions.

HOW
Add a +/- (default is -) in front of class name in permissions for the parser to determine whether an allow or deny set is to be created.
Example:

final String src = "java.lang { +Class { getName(); getSimpleName(); } }";
final JexlPermissions p = RESTRICTED.compose(src);