v0.5.1

New Features

• Add transmit function that returns the mailbox number, and transmit abort function (#25).
• Add more methods to acknowledge interrupts (#30).
• Add Can::free, a way to get back ownership of the raw peripheral (#33).

Fixes

• The Can::enable_interrupt and Can::disable_interrupt functions now manipulate the correct bits in the interrupt
  enable register (#29).

Misc

• Improve documentation of interrupts (#30).