Added support for I2C PCA9632 (4-bit LED dimmer)

Hi,

for my own demands I wrote the support for the I2C 4-bit LED dimmer PCA9632 from NXP. Now I'd like to contribute it to the community. This is my first big git action and my first pull request. I hope there are not too many mistakes. 

modified:   hardware/i2c/master/Makefile
modified:   hardware/i2c/master/config.in
new file:   hardware/i2c/master/i2c_pca9632.c
new file:   hardware/i2c/master/i2c_pca9632.h
new file:   hardware/i2c/master/i2c_pca9632_ecmd.c

Best regards,
Michael (Beroot)