* doc/api.md: fix a couple minor typos
Add commands to delete and update tags

Fixes https://github.com/puppetlabs/razor-server/issues/68
command update_tag_rule: require 'force' option if tag is used by any policies

If a tag is used by any policies, require that the user pass a 'force'
option to the update-tag-rule command
Evalute tags against all nodes on changes to the tag

* When creating or updating a tag, evaluate it against each node and
  applied/removed as appropriate. Evaluation happens asunchronously
* Updated api docs
Razor::Data::Tag: update node tags whenever a tag rule changes

This does not change the current functionality, but makes it more likely
that we'll keep the API promise we are making now that node.tags reflects
what existing tags apply in the future.