Update readme for windows, linux and macos (Fixes #35)

Updated readme for windows, linux and macos for mozmill-environment. 

Windows
- Shortened the line lengths, reduced lines to under 80 characters. Added a generic implicit path in the usage example. Removed the 'setup.cmd' line that was the source of the original issue.

Tested with Notepad, Wordpad on windows, and also Vim on windows, to make sure it reads nicely on all readers, has no line wrap problems, and there are no other issues.

Linux/Mac
- Replaced the phrase 'Interactive' with 'Manual' so all 3 readme's use consistent terminology. Corrected what I believe to be an extra character in the usage example for manual: ". ./run" to "./run". I could be wrong, but I don't believe any unix command runs when preceded by dot.