Fix #74 improve FastPin type declaration

[jfpoilpret]

Added 3 template type aliases to ease usage of gpio::FastPin in programs:

• gpio::FAST_PIN<board::DigitalPin>
• gpio::FAST_INT_PIN<board::InterruptPin>
• gpio::FAST_EXT_PIN<board::ExternalInterruptPin>

Also added 1 template type alias to ease interrupt::PCISignal usage:

• interrupt::PCI_SIGNAL<board::InterruptPin>

All aliases are uppercase as per FastArduino coding guidelines. All examples and tutorial samples have been updated to use the new API (better than old one, still present).

Thanks @hreintke for this request for a useful enhancement.