New functionality - file method to allow pulling raw file content from Zotero

This PR incorporates the "file" method to pyzotero that will pull the raw content of the file and return it as a binary string. This also includes the corresponding test and documentation update.

There is a limitation on the type of files that will be returned as raw content, as given by:

```python
        self.file_content_types = ['application/msword'                          
                                   'application/pdf',                            
                                   'application/octet-stream',                   
                                   'application/x-tex',                          
                                   'application/x-texinfo',                      
                                   'image/jpeg',                                 
                                   'image/png',                                  
                                   'image/gif',                                  
                                   'image/tiff',                                 
                                   'application/postscript',                     
                                   'application/rtf']                            
```

Related Zotero API documentation:

https://www.zotero.org/support/dev/web_api/v3/file_upload

Cheers!