Clean up the code and add null pointer checks

The commit cleans up the code, such that it compiles without any warnings.
Additionally it is ensured that no segmentation faults occurs. There have
been several different types of changes.

 - Unused variables have been removed.
 - A lot of cleanup and documentation has been added in the course of issue #2.
 - A numpy deprecation flag for version 1.12 has been set.
 - Attention has been paid to the difference between PyArrayObject and PyObject
   pointers.
 - Checks to detect memory allocation errors have been added, which would have
   otherwise resulted in segmentation faults.

Closes #7.