Removed Wire.endTransmission() for Arduino >1
Wire.requestFrom() will already send a STOP thus releasing the bus. (I also tried to keep the bus, so reads greater than the TWI/Wire buffer size would be simpler to write, but that didn't work yet.)

The removed Wire.endTransmission() started a new transmission by sending a START + ADDRESS and would send a STOP after receiving ACK. Without it you'll save a few microseconds.

readBytes() tested with the MPU-6050 DMP example on UNO using Arduino IDEs 1.0.1, 1.0.5 and 1.5.2.
readWords() tested with a simple demo for UNO reading values from another Arduino. IDE 1.5.2 was used.
Changed return description

It seems that the return value in case of an error is always negative.
(It should be OK to call this method to read zero words. Getting a zero
back shoud then indicate success.)
Fixed typo
Merge pull request #44 from snoeg/master

Removed unnecessary endTransmission() after requestFrom() for "ARDUINO > 100".