Update edXpsl.sty

Use `xparse` syntax consistently and `expl3` syntax appropriately. Alter coding style to adhere to LaTeX project conventions.
Merge pull request #8 from vermiculus/patch-1

Update edXpsl.sty
Added ToC referencing capabilities in main.py, render/edXpsl.zpts, plastexpy/edXpsl.py with supporting macros
Added the modified edXpsl.sty files for compiling course to PDF, located in the \texinput folder
Added a deleted comment to edXpsl.py file noting that edXsections become edXsequentials
Added documentation entries for toclabel and tocref, and modified some content for edXvideo and edXdiscussion
Modified main.py to prevent the TOC routine if running section_only, and removed duplicate lines from edXpsl.py
Modified main.py and edXpsl.py to correct PEP8 violations
Added starred versions of edXenvironments to plastexit.py, and modified test scripts based on PEP8
Added a test case for labeling and table of contents test_toc.py, with example11_toc_test.tex
Returned 2 deleted lines so error test passes on line 18 (not 16)
Modified test_toc.py to include more testing, corrected a few typos in latex2edx/main.py pertaining to MO tags
Modified edXpsl.sty files according to vermiculus suggestions
Modified edXpsl.sty files according to vermiculus suggestions, also reduced the number of pylint and pep8 violations in test.sh
Modified edXpsl.sty files according to vermiculus suggestions, also reduced the number of pylint and pep8 violations in test.sh
Fixed typo on line 641 of main.py: tags --> taglist
Fixed button generation for case when tocref has an unmatched toclabel
Corrected typo in button link to ToC, redundant label tag
Removed a line break and bad continuation
Cleaned up lines in excess of 80 characters in main.py, changed self.assert calls in test_toc.py, and updated edXpsl.sty with new macros after rebase