Adds the ability to filter invalid modules

Adds a constant that includes the names of modules that Meteor uses
which are not present in the npm registry. These modules are skipped
when building the package.json for the project.

Closes #70 #71 and #73

Includes fixes plucked from #73 (thanks @BigDSK <3).
Merge pull request #74 from onmodulus/bug/aws-sdk-browser-builder

Adds the ability to filter invalid modules