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