Lots of usability improvements for the I2C API. Better docs and new top-level functions. (#64) ...

* Lots of usability improvements for the I2C API.  Better docs and new top-level functions.

* Document frequencies

* Tabs to spaces

* More style fixes

* Run astyle

* Clean up docs

* Add note about addressing, change 10 bit to 11 bit

* Fix spellcheck

* Fix paste error

* Oops, fix accidental change

Refactor hal directory