Adding call() function with variadic templates

[adamcavendish]

For C++11 users, maybe we can setup helper call() functions with variadic templates instead of set_arg() stuffs?

For example:
Kernel.hpp
Kernel.tcc

[kylelutz]

Something like that would be interesting to have. Currently there is the variadic kernel::set_args() function which can be used to set all of the arguments for a kernel with a single function call like:

kernel.set_args(input.size(), input.data(), output.size(), output.data());