Added an Activity(int scheduler, int priority, ...) constructor

Otherwise, the contructor call `Activity(ORO_SCHED_RT, os::HighestPriority)` would interpret the second
argument as a double period time and ORO_SCHED_RT as a priority, which can lead to very strange and unforeseen
results.

Signed-off-by: Johannes Meyer <johannes@intermodalics.eu>
Merge pull request #74 from meyerj/new-activity-constructor

Added a new Activity constructor