Release 2.9.1

• Merge pull request #31 from francois-keith/master
• Use boost::pi rather than M_PI for portability.
• [Windows] Correct macro and export.
• No more debian in master branch.
• [Travis] Use jrl-travis submodule.
• Change the GripperControl entity. Close #27.
• Merge pull request #28 from mehdi-benallegue/master
• Correct the time incrementation for control recomputation
• Add a unary operator that computes the norm of a vector.
• Merge pull request #25 from mehdi-benallegue/master
• Make the velocity signal available even when the robot is not controlled by acceleration.
• Add const in the controller interface, remove useless semi-colon
• Merge pull request #26 from gergondet/topic/FixSignalValuesSetting
• Set signal values after integrate but before "after" periodic call
• Merge pull request #24 from mehdi-benallegue/master
• Set the signal values AFTER the integration
• [tools] Declares seqid in the pool of signals.
• [cmake] Synchronize
• [tools] Add seqid output signal to joint-trajectory-entity.
• Fix warnings.
• Merge pull request #23 from francois-keith/master
• Correct Derivator class.
• Add an operator to extract the position of an homogeneous matrix.
• Comply with modification of interface in SignalBase
• Merge branch 'master' of https://github.com/stack-of-tasks/sot-core
• Minor feature added in meta task posture.
• Minor patch in robot-viewer helper to accept the naming of the py:HumanoidRobot class.