Added tab-completion for yarp rpc when Readline library is available

(only works properly with Thrift-generated services)
When available, use Readline for all stdin operations [thanks Alexis Maldonado for the original patch]
Added proper FindReadline.cmake and use our CMake package-handling macros
yarp companion: Fixed EOF detection
yarp rpc: only ask command list to the server when tab completion is invoked
[thift] add help command itself to the list of commands reported by the default help function
yarp rpc with readline: complete "help a<TAB>" inputs