Add support for copying non JavaScript files to the coverage target directory to
the instrument command.
Add a test case for --complete-copy.
Add a test for no-complete-copy.
Less copy and paste, use constants.
Fix unused variable warning.
Use read and write stream when copying non JavaScript files and read file in
chunks.
Add test for backward compatibility.
Fix style consistency issue and use four spaces for a tab.
Add a docstring.
Also add 'error' event handler for the writeStream and move it infront of the
pipe call.

Also make sure 'callback' is only called once.
Merge pull request #62 from Kami/instrument_copy_non_js_files

[RFC] Support for copying non-JavaScript files to the instrument command output directory