Message Policies in XDI2

I implemented support for message policies in XDI messages.

This means that an XDI client can specify that a message should only be executed if certain conditions are made.

This is the "client-side counterpart" to XDI link contracts.

I.e. instead of the XDI endpoint enforcing a certain policy, the XDI client itself can send a policy along with the message.

See here for Drummond's description of this feature:

https://wiki.oasis-open.org/xdi/XdiMessagePatterns#ConditionalExecutionRules

In the XDI Local Messenger there's an example where you can try this:

http://xdi2.projectdanube.org/XDILocalMessenger?category=5&sample=1

It's a $mod operation that only gets executed if a literal value in the target graph is $greater than 80.

Markus