Counter for memory bandwidth (loads + stores)

Additionally one can now also measure load and stores, and thus the
memory bandwidth. Therefore, also the arithmetic intensity.

One caveat, though: The user is responsible to provide a meaningful
value for the amount of bytes transferred in one load/store, via the
"bytes_per_ldsr" parameter of the new function %set_print_options.

Till now, I have now way of obtaining this value programmatically, and
it also can and will vary for different sections of a program.

For example, a SSE movapd instructions loads/stores 16 byte, but is
still counted as one "load and store" instruction, just as well as a
1-byte mov. Feel free to advise me on a better set of machine counters..

Also, somewhat updated documentation.