Updated bigmemory Gallery. 

Example now shows how to correctly implement functions for big.matrix objects. Previously the object was only implemented for BigMatrix objects of type "double". Since we need to do some casting to get at the values in the matrix, it is not possible to enforce type in the function signature, so we need to handle the type checking at runtime. There is currently no documentation for this, so I have updated the Gallery post to rectify this, and provide a useful design pattern to follow when implementing generic functions that should work on a `big.matrix` of any type.