drivers: Initial import of SPI low-level driver IF

This PR is a alternative for #1309 and closer to the existing driver interfaces.

The proposed interface should act as ground for discussion - I am not yet sure yet if this interface will be sufficient...

As a little bit of a novelty it introduces further methods to power-on and power-down SPI devices. This functionality should maybe also be included in any existing low-level driver, since our goal for RIOT is always to be as energy efficient as possible!
Almost all devices I know use a 8-bit register addressing scheme. The (minority?) of devices that use different schemes can be serviced using the 'simple' spi_transfer_* functions...