ENH: add hdf5 io and AverageRFR IO capacities

as the name explains: put TFR objects in a fiff. Tests pass on my box.

cc @agramfort
cc @virvw
I know they're there -- they've actually been necessary for some analyses in the past (i.e., using other utils). They aren't redundant, if that's what you're thinking -- it stores all the fields we currently don't parse in `read_info`. So with this step, you are throwing away information that might be needed someday. 

Why did you want to exclude them here? Do they increase file size? Break IO?