[EI-3347] Add support for binary download

[andyantrim]

Issue
Currently we download all content as a string and store that string in the Data class. To prepare for additional file types, we should add support for binary downloads, without breaking existing usage.

Fix
Updated the data class

• Added as_bytes function, that returns the bytes content
• Added as_string function, which returns the string content of the file (as download did before)
• Updated download function, it just calls as_string and is marked as depreciated, and to be removed in 0.2.0

[vtermanis]

Add deprecation notice for docs:
(if this were to first appear in release 0.2.0, then)

        .. deprecated:: 0.2.0
            Use `as_string()` or `as_bytes()` instead

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[vtermanis]

1. Let's discuss what's best to do with the Data class tomorrow.
   • Your change is obviously in line with what already exists so this might be fine as-is but let's see what we want to do.
2. CHANGELOG.md should be updated with bits added for unreleased updates (yes - I know I've also forgotten to do so) - or we remove the changelog (since it's pointed to from PyPI) and use another method. I thought it'd be better to use this file than GH releases since that would make it GH-specific.

[vtermanis]

LGTM.
All the feedback I have are minor things - consider at your discretion