Add support for documents

[taylorgibb]

When sending a document this is the response, this adds a helper method like files and images have to get the document reference so we can download the attachment.

[taylorgibb]

Tested as below, works well, can we get this merged in so i can update our package from pip ?

[Kalebu]

Hi @taylorgibb

Thanks for taking your time to contribute

Merging this

[taylorgibb]

Thank you so much, much appreciated! Thanks for the work on this library as well!

[taylorgibb]

@Kalebu any idea when you will be able to publish this to pip ?

[Kalebu]

There was no definite like circle to publish to PyPI but I will definitely do it this week

Have two questions ASAP

1. Does this pull request also fix issue get_file function not implemented #45
2. Do you also have spare time to work on Add functionality to send_sticker #43?

Thank you @taylorgibb

[taylorgibb]

Yes it does fix #45 but i suppose i should update the docs and the sample hook as well. Ill send a separate PR for that later today as there is not concept of message_type file anymore, Whatsapp is now sending them as message_type document.

I can take a look at #43, i just want to get our project over the line first.