Add I2C driver and a 24LC024H i2c eeprom usage example
Now only support I2C1, not yet fully support I2C2

****GPIO Config****
I2C1 SDA -> PB8
I2C1 SCL -> PB9

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@1716 bbd45198-f89e-11dd-88c7-29a3b14d5316