Support for boost::thread and use of boost::async instead of std::async

Main author: Marcel Luethi <marcel.luethi@unibas.ch>
install vtk5 for travis (speed up compilation process)

* Fix superbuild when USE_SYSTEM_VTK is turned ON

* When system VTK is used, there should not be any dependencies for ITK target to
VTK's one

* compile ITK with VTK modules only if system VTK is at least vtk6

* travis machines have 2 cores available

    http://docs.travis-ci.com/user/ci-environment/#The-Build-Machine

Conflicts:
	.travis.yml
fix compilation for vtk5
Merge branch 'module' into boost-async2

Conflicts:
	modules/core/include/LowRankGPModelBuilder.h
Make Windows find Boost
Merge pull request #92 from statismo/windowsfix

Make Windows find Boost
Merge pull request #89 from statismo/boost-async2

Support for boost::thread and use of boost::async instead of std::async (develop)