Added version number to pystencils

Pystencils should have a __version__ attribute

pystencils/__init__.py

 """Module to generate stencil kernels in C or CUDA using sympy expressions and call them as Python functions"""
+from pkg_resources import get_distribution
 from . import sympy_gmpy_bug_workaround  # NOQA
 from . import fd
 from . import stencil as stencil
@@ -21,6 +22,8 @@ try:
 except ImportError:
     pass
 
+__version__ = get_distribution('pystencils').version
+
 __all__ = ['Field', 'FieldType', 'fields',
            'TypedSymbol',
            'make_slice',