Add PySide2 to setup.py under view key

[boredstiff]

PySide2 is on PyPi as of a week ago.

This adds the PySide2 dependency underneath the 'view' key in extra_requires, as discussed in #269 (comment)

Updated the README with instructions as well.

[codecov-io]

Codecov Report

Merging #293 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #293   +/-   ##
=======================================
  Coverage   90.92%   90.92%           
=======================================
  Files          49       49           
  Lines        4352     4352           
=======================================
  Hits         3957     3957           
  Misses        395      395

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[boredstiff]

(otio) C:\dev\OpenTimelineIO>pip install -e .[view]
Obtaining file:///C:/dev/OpenTimelineIO
Collecting PySide2==5.11 (from OpenTimelineIO==0.8.0.dev1)
  Downloading https://files.pythonhosted.org/packages/fe/4e/06a26bb928fabb6374d85532c8bdeee1c17a64e683f5104d9698fa84c98e/PySide2-5.11.0-5.11.0-cp35.cp36-none-win_amd64.whl (129.5MB)
    100% |████████████████████████████████| 129.5MB 218kB/s
Installing collected packages: PySide2, OpenTimelineIO
  Running setup.py develop for OpenTimelineIO
Successfully installed OpenTimelineIO PySide2-5.11.0

[jminor]

Thanks!