Distinguish 'x' and b'x' in Python 2

[ilevkivskyi]

Fixes #10

@gvanrossum @JukkaL @ddfisher
This is a naive fix. It only checks the first string in concatenation at CST level, for b or B prefix and sets the has_b flag in Str AST node accordingly.

[gvanrossum]

I am happy with this. @ddfisher, would you mind merging this once you're back from your vacation? It won't require mypy changes (but it enables some that I am going to work on).

[gvanrossum]

Oh, before merging, please update version in setup.py.

[ilevkivskyi]

@gvanrossum
In my other PR #16 I propose to change that will expose the version, see https://github.com/dropbox/typed_ast/pull/16/files#diff-7e54a5009e78743c7e43425c51351356R1 and https://github.com/dropbox/typed_ast/pull/16/files#diff-2eeaed663bd0d25b7e608891384b7298R87

[gvanrossum]

Yeah, if both get merged around the same time we only need one version increment. It's up to @ddfisher at this point. (David: note that GitHub added a feature to allow reviewers to edit PRs in the web UI. Very cool!)