Added script that extracts all data tables from specified database.

This is a script that I use in my overall project that makes use of the macarthur_core/lovd and macarthur_core/web_io to extract data from all data from a given LOVD URL. 

The script makes use of argparse to provide a user interface. The string provided for the command-line interface should hopefully provide an explanation of how it is used. It should save a tab-delimited file for each gene's table data, where each output file is named according to the gene name.

From command line generally it is used in the following way:
```
python extract_data.py --all --leiden_url http://www.dmd.nl/nmdb2/ --output_directory results
```
Users can also print a list of all available genes using:
```
python extract_data.py --genes_available --leiden_url http://www.dmd.nl/nmdb2/
```