Expose _.now

Expose the internal function getTime as _.now and alias _.getTime
Merge pull request #1410 from megawac/now

Expose _.now
more concise implementation of _.now/_.getTime
Remove _.getTime alias, comment.
Faster _.now fallback.
Update CONTRIBUTING.md
update link to underscore.string

(Official repo was moved to https://github.com/epeli/underscore.string)
Merge pull request #1415 from skv-headless/master

update link to underscore.string
use `contains` instead of `indexOf` in _.intersection. closes #1417
use _.partial in definition of _.intersection

This takes the tweak suggested in #1417 one step further.
Merge pull request #1418 from davidchambers/intersection

use _.partial in definition of _.intersection
Refactor _.partial.
Cache length.
Merge pull request #1422 from braddunbar/partial-refactor

Refactor _.partial.