enabled setup to use system cfitsio

I have added an option to use the system cfitsio distribution instead of recompiling. This option is turned off by default, but is a nice build feature to have. You can pass the option `--use-system-fitsio` directly to the `setupy.py` script. You should also be able to invoke it with `pip` via 
```python
pip install fitsio --install-option="--use-system-fitsio"
```
I know this breaks the bundling rule, but it gives users more flexibility when compiling `cfitsio` locally fails using the default `make` invoked in `setup.py`.