FrSKY SmartPort UAVO bridge

I've created support for FrSKY S.PORT telemetry protocol (used by new FrSKY receivers and Taranis). It supports and transmits almost all telemetry values supported by Taranis - there is no freely available documentation of the protocol so I just studied openTx (Taranis source code; receiving side). 

One of the commit of this PR is also enhancement of STM32F3 USART driver to support single-wire inverted mode. It enables F3 targets like Sparky and flyingf3 to be able to interface with FrSKY receiver directly without any external circuitry. For other targets, circuit like this is required:
![sport](https://cloud.githubusercontent.com/assets/2689640/3487545/c723bd82-0486-11e4-96b1-5a9be8c3bed8.png)
I'll add this diagram along with some description to the wiki when (and if) this merges.

Some quick demonstration video:
https://vimeo.com/99997249