* Changed time units as outlined in issue #2353.
Also applied changes to to_str for Unit - not sure if that was what was wanted.
* Forgot the tests!
* Updated primitive.rs to match changes.
* Updated where example to match changes.
* And the html test!
* Rename `calc` to `math eval` and allow it to optionally take the expression as an argument
* Moved calc tests to math eval
Also added 2 tests and changed 1 test
* Move calc docs to math eval