Consistent & correct handling of integers and gradients

-Documentation and implementation of a consistent way of handling gradients and integers
-Type checks that ensure the gradient is always floating point and not an integer
-Type checks that ensure the gradient of an integer is always undefined or 0
-An upgraded version of connection_pattern that provides theano with enough information to answer questions like "is variable x a function of variable y?" accurately

fixed an error message

added doc and argument checking to MRG_RandomStreams.multinomial

Mixed

Try to clarify points raised during code review

deal with the case when the grad on one output is undefined

It get inherited from the CPU version. DebugMode will still work correctly as the op is inserted by an optimization.