Renamed authors.txt, license.txt and readme.md to uppercase variants

This sets the text files apart from the rest of the code files and seems
to be the standard for most projects
Converted AUTHORS and LICENSE files to Markdown

.txt files are rendered by Github as plain monospace text. Markdown files
however are rendered with a more readable font and formatting.
LICENSE: Fixed reference to AUTHORS file
test: Renamed readme.md to README.md

.. and updated link in main README.md

(Github supports relative links now AFAIK)
test/README: Updated "make test" to "./build.py test"
Added notes that "./build.py test" requires plovr to be running