HOTFIX: disable interrupt-driven I2C mode, configure pessimistic I2C timeout, correct handling of the NAK generation for I2C master reads.

This looks like it addresses the recent I2C lockup issue, unfortunately it also increases CPU consumption by ~5% for the I2C sensor bus.
Bringing back old I2C code, just temporary workaround
First round of testing successful - back to I2C code from Dec / Nov 2012
Merge pull request #168 from PX4/i2c_workaround

I2C driver workaround for NuttX
Merge branch 'master' of git://github.com/PX4/Firmware into fault_detection