* Introduce new operation with suffix "IfExists" for all
string and numeric condition operations;
* An operation with "IfExists" suffix specifies the following: if the condition key
is present in the context of the request, process the key as specified in the policy.
If the key is not present, evaluate the condition element as true;
* Change the signature for interface method `Property()`: since it returns two
values - the second indicates whether the property exists;
* This also means that original condition operations for string and number comparison
is slightly changed: if property doesn't exist, then the condition is not evaluated.
Signed-off-by: Airat Arifullin <aarifullin@yadro.com>
