Add docs, code coverage and tests to .npmignore

Cleaning up the .npmignore for Arrow. The package that gets installed by users has a bunch of files that are test files/docs. Adding these to npmignore also brings down the size of the package by 4.6MB
```
CHANGELOG.md
LICENSE.txt
README.md
arrow_selenium
arrow_server
config
coverage
demo
docs
ext-lib
index.js
lcov-report
lcov.info
lib
lib.js
node_modules
nodejs
npm-shrinkwrap.json
package.json
results.xml
sharelib
tests
tmp
ytestrunner.json
```