Add .npmignore files to exclude test and source files

[Satyam]

Currently all the test and source files are installed along the transpiled files in /lib, which is, I believe, all the developer needs.

Since the packages exist as separate entities in npm, I am not aware that the current root .npmignore has any effect at all. Npm searches for .npmignore files at the project root (in this case, each package root) and its subdirectories, but it does not search parent directories. This can be tested via npm pack. The command lerna exec npm pack will create tar-files in each package root to prove this.

As stated in this Lerna issue it is better to place .npmignore files in each package.