Using external Javascript library is not so easy... Depending on the context in which you want to use it (a webapp of a document embedded script) the approach will be different.
I didn't try it in client JavaScript and I'm not sure caja will allow it but I found this post that shows a possible way to include it using a Google Script Library that a user has build and if I read the post it seems to work...
The "user" is a Google developper so he knows for sure what he is talking about ;) please update here if it worked for you.