Basic DAC Driver, implementation on stm32f4discovery

- Creation of DAC Driver, much like the current ADC Driver.
- Implementation of the DAC Driver for the stm32f4discovery board.

dac_init(dac_t dev, dac_precision_t precision)
Initializing a DAC Device with a given precision, configures the Device and Channels. 

dac_write(dac_t dev, uint8_t channel, uint16_t value)
Writes a value on a specific channel for a device.

dac_poweron(dac_t dev)
Enables a given DAC Device.

dac_poweroff(dac_t dev)
Disables a given DAC Device.

uint16_t dac_map(dac_t dev, int value, int min, int max)
Maps a integer value between interval min and max, to a valid integer to write onto the dac with respect to the selected precision.

uint16_t dac_mapf(dac_t dev, float value, float min, float max)
Maps a float value between interval min and max, to a valid integer to write onto the dac with respect to the selected precision.
Travis is unhappy:
```
cpu/stm32f4/periph/dac.c:86: style (unreadVariable): Variable 'dac' is assigned a value that is never used.
cpu/stm32f4/periph/dac.c:87: style (unreadVariable): Variable 'val' is assigned a value that is never used.
```
Maybe you could try to just initialize them without assigning a value?