Replace encoding override with improved stdio wrappers; use unicode_literals

This further develops our Python 3.3 migration strategy, most importantly Unicode handling.

    - Use "from __future__ import unicode_literals" everywhere.
    - Use "sys.stdin = io.open(sys.stdin.fileno(), encoding=...)". This makes sys.stdin behave like it does on Python 3.3, producing unicode without having to rely on hacks, implicit coercion, or external configuration to fix the Python encoding.
    - Replace sys.stdout and sys.stderr using a subclass of io.TextIOWrapper. This subclass makes sys.stdout and sys.stderr accept Unicode text as they do on Python 3, but if non-Unicode input is given to .write(), it will try to cast it to Unicode. This provides backward compatibility, and is necessary to make errors show up on sys.stderr on Python 2.
    - Don't wrap the streams more than once.
    - Use an unbuffering sys.stdout wrapper for raw_input() on Python 2, since it doesn't know how to flush the wrapped stdout buffer before waiting for stdin.
Eliminate unused imports in the test suite
Decode test subprocess output properly
Use unicode literals in dxclient test suite
Set PATH for tcsh test
PTFM-11399 Workaround for test coverage instability.
Lock down test requirements; migrate from pexpect-u (deprecated) back to pexpect
Update Python boilerplate and add future imports to more modules
Pass bytes, not strings, as empty return values
io.BytesIO doesn't like mmap
Encode input to httplib to avoid Unicode coercion in Python 2
Coding conventions from internal wiki
Update Readme.md
Account for when outputFolder is not returned
PTFM-11584 Don't implicitly describe users in dx describe.

In order to describe a user user-alice you now must write "dx describe
user-alice" ("dx describe alice" no longer works so as not to cause the
requesting user to exceed the rate limit for querying for nonexistent
users).
PTFM-11399 2nd try: need to call scripts as processes to get coverage
Test coverage improvements
Changelog and version updates for v0.93.0