* 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.
Exclude the `vendor` directory from coverage analysis

When we run our spec tests using a bundler installed collection of gems, they
wind up under the `vendor` directory.  This was included in coverage analysis
by default, which included a whole pile of different code that we don't care
about in our reports.

This restructures the excludes and adds the vendor directory to the filter
list, so coverage doesn't touch it any longer.  No functional changes beyond that.

Signed-off-by: Daniel Pittman <daniel@rimspace.net>
Add policy_add_tag and policy_remove_tag commands

Adding the ability to associate/disassociate tags with a policy.  These
actions are idempotent in that adding or removing a tag that is already
associated/disassociated does not contitute an error.

Tests included.