[C++] Raise exception when inertia information is missing when build tree with a specific root joint

Functions defined in headers urdf.hpp and jacobian.hpp are set to inline.

The functions should be inlined, otherwise it breaks with the headers
are used in multiple robots mode.

Set default initialization of variables + expose spatial quantities methods + correct bug

[C++] Build sample humanoid model with joint limits.

  * Velocity and torque limits are choosen randomly from the [0;1]
    interval for 1D joints.
  * Position limits are choosen randomly from the [-1;1] interval for
    1D joints.
  * Torque limits set to zero(6) for freeflyer joint.
  * Velocity limits are choosen randomly from the [0, 1e3]^6
    interval for freeflyer joint.
  * Position limits are choosen randomly from the [-1e3, 1e3]^7
    interval for freeflyer joint.

[Python] Add StdVec::index binding to emulate python list behavior.

  * It is defined only for string vectors for now.

Fix URDF inertia parser

  * Inertial-less links under fixed joints exist for semantic
    purposes, i.e. to provide referential frames.
  * Keep storing these links as fixed bodies.
  * The case of pairs with non-fixed joints and inertial-less links
    still needs to be properly addresses, as the current behavior
    (identity inertia) is clearly bad.

Fix some bugs + update Mac compatibility

May need some change according to the current version of the
architecture

[C++][Bug] Move sample-models function implementations out of header.

  * One Definition Rule was still not enforced.
  * Declare function in header sample-model.hpp, implement them in
    sample-models.cpp
  * Add sample-models symbols to libpinocchio.

create model using python & (fix) exposure of joints

[Python][Major] Binding Model::addBody. Be careful, unless C++ method addBody, in python there is no default parameters

[Major][C++][Python]Breaking retrocompatibilty. Added the possibility to choose root joint when creating a model (in C++) or a robot (in Python). May be a check can be done on the value of root parameter in python if it's not None

[CMake][Bug] Fix use of lua sources cmake variable.

Force Pinocchio shared library + move gravity vector to the shared library

[cmake][C++][bug fix] Moved initialization of static const variable gravity 981 from model.hpp to model.cpp. libpinocchio.so is no more empty. Caution : Pinocchio is more a header-only library. One should alwais link against shared library

[travis] Give travis more time for executing full build

Enforce (again) One Definition Rule