closes zeit/ms#62

* throw an error if the val is invalid

- give an error response to developers immediately, when calling `ms(null)` or `ms(undefined)` or other invalid things.
- to help developers avoid mistakes, and less debug.

* add test cases

Update all dependencies 

Closes #45

Rename `long` and `short`

Cause it is reserved words in ES1-3. It is ok for ES2015, but it still breaks down Closure Compiler.

Specify license on package.json

Hey there,

We're using an automated tool called License Finder (https://github.com/pivotal/LicenseFinder) and your node module coms up as "Unknown". By adding the license to package.json you would help immensely users of this and other automated tools to figure out whether a module can be used inside another project.

Thanks!

Fixed broken readme link