Only import NLTK and other modules when they are needed.

[davidbaines]

Would it be practical to move the importing of NLTK and other large imports so that they are only imported when required.

Move slow imports to where they are needed.

1. Not every script needs NLTK and checking that it has been downloaded slows every script.
2. Maybe there are other 'slow imports' that could be moved so that they are loaded only when required.