Polish the various utility scripts + fix bugs

[pv]

Address the following problems:

• No scripts get installed on PATH, so they're annoying to use.
• makestatic.py and extractsagecode.py don't work correctly with current Pyparsing versions
• Scripts assume Sage is found at ~/bin/sage, Latex files are in cwd, etc.

So:

Add a sagetex-tool script that can be installed on PATH (unlike makestatic.py etc. whose names are not suitable for that). Make sagetex a python package, make the scripts modules inside it, and make setup.py to install sagetex-tool as the command-line entry point. Do this in a way such that all scripts can still be run standalone, and you can also use sagetex.py as a Python module (instead of a package) exactly as previously if you like.

I guess the scripts could also instead be renamed to sagetex-makestatic, sagetex-run etc., so let me know what you think. Ok, it's simpler to just rename the scripts as sagetex-XXX and mark them and the sagetexparser.py to be installed in setup.py.

Fix some brokenness and inconveniences. This includes a commit cherry-picked from #55 which makes makestatic.py to work again with current pyparsing versions. extractsagecode.py also needed some fixes.

The code assumes Python3 only.

[dimpase]

Sorry, I missed this PR back in April, as for some reason notifications on this repo aren't working for me.

Thanks a lot for contributing this.