Refactoring of internal APIs and adding LISTING_STATUS function

[dhoeppe]

This is my first major open source contribution to a project, so please forgive me if there are flaws.

I basically wanted to add support for the LISTING_STATUS function of the AlphaVantage API. Due to the fact that the API returns the data of this function as CSV only it was required to override the output_format specified at object instantiation.

I tried using the existing decorator, but these were, to my knowledge, unfit to accept arguments. It was therefore required to create decorator classes that are able to accept arguments. As I had no experience with this library I refactored large parts of it while doing so.

Related issues would be: #290, #287, #283, #269, #260

A short overview of what was done:

• Decorator overhaul: The function decorators had an override parameter, but this could not be set. I could not find a code example of its usage. I figured to set the parameter it was required to create a decorator class. The _output_format decorate overrides the output_format parameter of the AlphaVantage object.
• Usage of requests.Session: There was a lot of string manipulation to create the URL invoking the API. Using the Session class this could be reduced.
• Code duplication: There was some code duplication between async_support and the synchronous code. I tried reducing this as much as possible.

I hope this gives a good overview of the changes. Please don't hesitate to ask any questions.

[dhoeppe]

Any new information regarding the processing of this pull request?

I have published the fixes together with support for rate limiting on PyPI until this PR is processed further, check out this repository for more information.

[AlphaVantageSupport]

@dhoeppe , thanks for your time and contribution! Unfortunately, the build failed after merging the pull request and we had to revert the merge. Below is the build detail:

https://travis-ci.org/github/RomelTorres/alpha_vantage/builds/774491793

We would appreciate it if you could address the build issues - feel free to submit a new pull request. Thank you!