Install using api not working and other fixes

[dhoer]

Fixed the following issues:

• The examples on the readme to install matlab2tikz for tag, commit, and branch don't work. The api download is only for tags. I added a fix for this and as well as tests.
• Readme was missing info about --approve flag.
• Example downloading from fileexchange is not working because download link points to github. Replaced example with covidx package example that does have download link to fileexchange. Added test for fileexchange.
• Added tests for search, freeze, install via git clone, and infile
• export_fig uses git url in requirements-example.txt, since fileexchange doesn't work

[dhoer]

BTW, thanks for the quick turn around on the --approve flag.

[mobeets]

Great!