Add support for struct arguments

Problem: 
	- Sending a message to a mock with an struct argument produces an error: "-- Unhandled type: {CGRect={CGPoint=ff}{CGSize=ff}}"

Fix:
	- Detect struct arg in TKArrayArgumentsForInvocation.
	- Allocate memory to store struct. Important: initialize with zeros to discard alignment padding.
	- Wrap struct in NSData and store in the array.
	- Free allocated memory. Should be safe as long as dataWithBytes:length: copies the bytes (as explained in https://developer.apple.com/library/mac/documentation/cocoa/Conceptual/BinaryData/Tasks/WorkingBinaryData.html).

Provided tests:
	- Verify tests that show how structs can be passed as args to mocks.
	- Stub tests that show how structs can be used for stubbing. 
        - Note that equality check compares the bytes inside the structs.

Possible improvements:
	- Method dataWithBytesNoCopy:length: could be used instead of dataWithBytes:length:, that should remove the need for freeing arg pointer.