Use fully qualified names in function and stencil definition macros

fixes #22
Fixed extract_user_function transform for nested functions

closes #21
Merge branch 'master' of https://github.com/ddemidov/vexcl

Conflicts:
	examples/utests.cpp
Revert "Allow arbitrary expressions as FFT input."

This reverts commit f23ce5ccdc3c95f7f5ca3017e9a8c1b99a129fee.

Conflicts:
	examples/utests.cpp
Allow arbitrary expressions as FFT input.
Result can only be added to a buffer and multiplied by a scalar though.