Added a new Activity constructor

This patch adds a constructor with the signature
```cpp
Activity(int scheduler, int priority, base::RunnableInterface* r = 0, const std::string& name ="Activity");
```
to the `RTT::Activity` class.

Otherwise, the contructor call `Activity(ORO_SCHED_RT, os::HighestPriority)` would interpret the second argument as a double period time (= 99 seconds) and `ORO_SCHED_RT` as a priority (= 0), which can lead to very strange and unforeseen results:
```cpp
Activity(int priority, Seconds period, base::RunnableInterface* r = 0, const std::string& name ="Activity");
```