Expose methods to conver from cl3 types to opencl3 types

[AlsoSylv]

This PR provides the ability to convert from cl_context and cl_queue from cl3 to Context and Queue in opencl3.
This is can be done either with the now public new method, or the respective wrap functions that take &Device instead of cl_device_id and max_work_item_dimensions respectively

Closes #73

[kenba]

Thank you, @AlsoSylv.
Can you please add some tests for the new functions?

[AlsoSylv]

Sure, what kind of tests are you looking for?