Fixed error from gen_js.sh when passing --lang into mavgen.py: switched to "JavaScript" from "javascript". Was breaking when following mavlink/pymavlink/generator/javascript/README.md instructions to build using "make -f javascriptMakefile". The output error was:

 ./gen_js.sh
 Usage: mavgen.py [options] <XML files>

 mavgen.py: error: option --lang: invalid choice: 'javascript' (choose from 'C', 'CS', 'JavaScript', 'Python', 'WLua')
Fixed require statements in test code which incorrectly included ".js" extension to implementation library.
Merge pull request #117 from cbumgard/master

Fixes for JavaScript (Node.js) generator build & tests