remove, rather than add, leading slash from tf frame name

This is in accordance with both the hydro migration guidelines [1] and the implementation of TFClient.subscribe() (TFClient.js:102)

[1]: http://wiki.ros.org/hydro/Migration#tf2.2BAC8-Migration.Removal_of_support_for_tf_prefix
Merge pull request #79 from T045T/devel

Fix TFClient by removing, rather than adding, a leading '/' character in TFClient.processFeedback()