Bridge: Fixed File "duplicate definitions" when using SD and Bridge libraries together

The two File classes have been enclosed into different namespaces.

To guarantee compatibility with old sketches that uses only one of the two
libraries an additional line:

   using namespace xxxxx;

has been added so the users can still use "File" where there is no ambiguity.

BridgeLib::File and SDLib::File classes have been also aliased to BridgeFile
and SDFile respectively, users are encouraged to use that instead of File.
Bridge: Renamed YunClient/YunServer to BridgeClient/BridgeServer

This makes the library more consistent and not tied to a specific board.
Bridge: Added compatibility layer with older YunClient