Allow planning groups to have more than one tip

*Slightly re-factored PR*

**Note:** *This commit will not build until the pending PR for moveit_core is merged ros-planning/moveit_core#149*

Adds functionality for moveit_ros to reason with kinematic solvers that have multiple tips, ie kinematic trees.

* Modifies KinematicsPluginLoader to check the rosparam server for an optional array of tips per kinematic solver. This is the recommended way to enable a kinematic solver to have more than one tip frame. Creates a vector of tip links to planning groups, instead of just a string.
* Moves the logic for choosing a planning group's tip frames into a separate helper function, to make it clearer and support multiple tips
* Removes the assumption in robot_model_loader that a joint model group can only have a kinematics solver if it is a chain