Initial import of low-level GPIO driver interface

Here is a proposed interface for a low-level GPIO driver interface.

For each platform the GPIO devices (one for each pin) are statically assigned to a specific pin in the 'periph_conf.h' file (which needs to be defined for each board). This can for example look like this:
GPIO_1 --> PA.2
GPIO_2 --> PB.14
GPIO_3 --> PD1 ... you get the picture.

This approach has the benefit, that pins used by applications are completely independent of the underlying platform, so code using this interface is hardware independent and portable. 

For now there are 15 GPIOs defined (if a platform has less pins defined, it can disable them by undefining the corresponding GPIO_x_EN in the boards 'periph_conf.h' file).