Ignore Webstorm IDE related generated output

[glorat]

I'm using Webstorm for development. It tends to write output into the source directories. This ignores them.

[coveralls]

Coverage remained the same when pulling d4b43e8 on glorat:webstorm into cfcfe59 on bitcoinjs:master.

[dcousens]

This is not the typical approach for handling .gitignore files in repositories.

If these files are frustrating for you, perhaps you should look into using a global gitignore.

Ideally the project local .gitignore should only ignore files generated by the project itself.