Release v0.0.15

[bug fix] BREAKING! Fix a bug with the main routine which was no longer able to build with updated versions of rust. This change removes the extern "C" directive from the panic handler.
[bug fix] No longer reserving pins 7 and 8 for kernel purposes
[feature] Add begin_with_external_power to i2c which will assume an externally-biased pull-up. Useful if you are working with less-than 3v3 components.
[chore] Add a new F_CPU variable and CLOCK_CPU to help with precise timing
[chore] Give some pin enums the ability to be cloned