updated L3G4200D::getAngularVelocityN() functions

I was trying to use this library but the raw values from the gyro where always positive, so I check the code and I found two mayor problems, first this call "if (getEndianMode() == L3G4200D_BIG_ENDIAN) {" re-use the buffer so the code below (where the function actually returns gyro values) produces values that are obiusly wrong, and the second thing I notice was that the I2Cdev::readBytes call just return the L3G4200D_RA_OUT_N_L register twice (L3G4200D_RA_OUT_N_H is was not being readed), so just half of the data was being received, so I try to fix it and it works, so if you could take a look and make something better it would be greate for people who are trying to use this gyro with this library.