Expose getSignals() in RWSInterface.

[Maarten de Vries]

PR #63 adds RWSClient::getSignals() to retrieve all signals from a controller. However, it doesn't expose the function in RWSInterface yet. This should still be done.

Quoted from @jontje:

Do you also intend to expose the new function in the more user-friendly RWSInterface class? For example:

* Implementing a `struct` for an IO-signal list item according to the [documentation](http://developercenter.robotstudio.com/blobproxy/devcenter/Robot_Web_Services/html/ios_signals_get_page.html), which contains:
  
  * name: IO-signal name
  * type: Signal type {DO | DI | AO | AI | GI | GO}
  * category: Signals list item
  * lvalue: Signal value
  * lstate: Signals state {simulated | not simulated}

* Parsing the XML document into a vector of such IO-signal list items, similar to [RWSInterface::getRAPIDModulesInfo(...)](https://github.com/ros-industrial/abb_librws/blob/master/src/rws_interface.cpp#L233-L250).

If not, can you open an issue for that?

Can you also update the PR to be in sync with the base branch?