Adding ROS namespace manipulation operations

[jbohren]

This aims to resolve issues mentioned in orocos#35

This allows us to do the following (in ops for example):

ros.setNS("/controllers/left")
// call function which creates other components
// all ROS names will be scoped to "/controllers/left"

ros.setNS("/controllers/right")
// call same function which creates other components
// all ROS names will be scoped to "/controllers/right"

ros.resetNS()

Or, if you have multiple deployers, you could run this code in each of them, without having ROS namespace collisions (unless you do something with an absolute namespace):

ros.pushNS("deployer_1")
// create a bunch of components
// all ROS names will be scoped to "deployer_1"
ros.popNS()

@smits @meyerj @konradb3 @cpaxton What do you guys think?

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This change is

[ahoarau]

@meyerj To my opinion, this would be a very useful addition to rtt_ros.

[meyerj]

Re-opened as orocos#141.