Fix some problem when installing under pypy.

Make get_devices() return an empty list instead of erroring out (reported by Tomasz Rybak on the master branch)

Conflicts:
	.gitignore
	src/wrapper/wrap_cl.hpp

Use the documented struct.calcsize to determine machine word size for better pypy support.

FIX: no more explicit padding required for vectors

Additionally, relying on default values for initialization triggers a
deprecation warning. Added three more vector constructors: filled_ which fills
the entire vector with a given value, zeros_ and ones_.

Closes #28