Add test function so tests can be run from within python terminal

* PR
```python
>>> import csympy
>>> csympy.test()
============================= test session starts ==============================
platform linux -- Python 3.4.1 -- py-1.4.25 -- pytest-2.6.3
collected 67 items 

miniconda3/lib/python3.4/site-packages/csympy/tests/test_arit.py .............
miniconda3/lib/python3.4/site-packages/csympy/tests/test_functions.py ...
miniconda3/lib/python3.4/site-packages/csympy/tests/test_integer.py .....
miniconda3/lib/python3.4/site-packages/csympy/tests/test_matrices.py ...............
miniconda3/lib/python3.4/site-packages/csympy/tests/test_subs.py ....
miniconda3/lib/python3.4/site-packages/csympy/tests/test_symbol.py ..
miniconda3/lib/python3.4/site-packages/csympy/tests/test_sympify.py ......
miniconda3/lib/python3.4/site-packages/csympy/tests/test_sympy_conv.py ...................

========================== 67 passed in 0.75 seconds ===========================
True
>>> 
```
Why is the `-p` needed? The `empty` directory should not be there before this command is called.