Enable saving to package.json and new symlink

"bin/onpm" has been renamed to "bin/onpm.js" to allow editing
like other javascript files.

A new symlink, "bin/onpm" has been created to point to the renamed
script file. This will allow invocation with `./bin/onpm`.

User will soon be able to install package and have it saved to their
`package.json`.

A new module `pretty-data` has been added to "package.json". It
facilitates printing JSON output to `package.json` in a pretty format.

Tests in "test/test.cache.js" have been slightly modified to reflect
the API change with `.installFromCache()` in "lib/cache.js". A new
test to check if the package.json is modified correctly has also been
added.