btle: readability improvements, more value reading, py2.7 compatibility

Hi,

your code looks like the best BLE-python interface so far, so I toyed around with it a bit and tried to make the btle output more readable. Feel free to cherry-pick any or all of the patches, or to write inflammatory comments about my bad coding style :)

I am not 100% sure about the short-form UUID patch, as it discards the first four hex-digits, which might or might not be zero in the spec.