Getting the contents of a signature request file

[arshsingh]

The get_signature_request_file method takes a filename, fetches the contents from the server and writes to this filename. This is a problem if the file is not stored locally (S3 in my case).

Potential solutions:

1. Make get_signature_request_file also accept a writable file-like object.
2. Make the filename parameter optional, and in it's absense return the contents as a string ?

I'd be happy to submit a PR if we can decide on a solution. I prefer the first one above, unless anyone else can propose a better solution.

[benjdezi]

I think the first solution is best, feel free to submit a pull request for it.