Python 2.7 Support - Closes Issues #128, #135, #138, and #157

[cyon1c]

This PR will bring Python 2.7 Support to steem-python.

Changes roughly fall into 2 broad categories:

1. Remove Breaking 3.6 Syntax

   • Type Hinting -> removal of type hints from method signatures.
   • Dictionary Unpacking/Flattening -> removal of ** syntax in dict construction.
   • Changing yield from syntax to for x in xyz: yield x.
   • Class definitions & super updated.

2. Type Compatibility

   • Bytes: in Python 2.7, bytes is simply an alias for str, which caused wide ranging issues. To resolve this, the future_bytes method was added to utils.py, along with the library python-future. All class methods named __bytes__ have been refactored to to_bytes and all references in the app to bytes() has been refactored to future_bytes.
   • Chr: chr() in 3.6 maps to unichr() in 2.7, so version checking was added in utils.py to call the appropriate function in the form of to_chr.

Currently, steem-python passes all tests (65/65) when run both in Python 2.7 and Python 3.6

[roadscape]

Some feedback after a cursory look:

• reviewing large refactors becomes difficult when bundled with changes in whitespace; ideally styling changes are applied separately. prefer to keep critical diffs minimal
• would it make sense to keep __bytes__ methods (instead of renaming to_bytes) but continue to call them explicitly? could make the intent more clear.
• please add some brief comments to each method such as future_bytes explaining the compatibility issue it solves

[cyon1c]

I've made updates which should address all your comments, @roadscape . @sneak and @john-g-g , how are we addressing the proposed changes for the pipfile?

@john-g-g regarding the updates to the circle.yml file, it just runs the docker build - so really do the updates need to be in the Dockerfile, not circle.yml file?