This removes byteorder as it is no longer needed (now provided by std).
Also no longer needed is extern crate.

This makes it easier to add the changes for the configuration register.

This allows users to change the way the INA219 operates.

This makes the coding style consistent.

Not sure yet about the calibration calculations...

Also add reading of registers to driver

This avoids looking up the pin mappings in the datasheet every time.

Added documentation, TryFrom and nicer error

This should allow zero-cost usage of the uncalibrated device.

This makes it so that only the errors that happen in a function can also
be returned and handled.

This makes the docs a bit more structured.

This should give us more confidence about future changes to not break
anything.

This should make it easy to later update to the full eh1

This introduces a feature to make all the paranoid checks optional at
 compile time. So users who do not care can just not have the overhead.

This allows users to retry initialization, because we are now not just
dropping the I2C device we were passed.