import changes directory

Importing the arrayfire module changes the working directory. For example:

```
$ python
Python 2.7.13 (default, Apr 4 2017, 08:46:44)
[GCC 4.2.1 Compatible Apple LLVM 8.0.0 (clang-800.0.42.1)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> print os.getcwd()
/private/tmp
>>> import arrayfire as af
>>> print os.getcwd()
/usr/local/Cellar/python/2.7.13/Frameworks/Python.framework/Versions/2.7/Resources/Python.app/Contents/Resources
>>> print af.get_version()
(3, 3, 2)
```

This causes problems with any file names specified with a relative path.

I'm running on OS X 10.11 with arrayfire 3.3.2 installed with homebrew and arrayfire-python installed with pip.

I don't see any `chdir` calls in the module, so I'm not sure where the problem is.