Add `Criteria#<<(criterion)` to stop repetition

Prior to this change `Nexpose::DynamicAssetGroup#criteria` accessor was
used to get the DAG's criteria, `Criteria#criteria` was then used to
get the actual array following `Array#<<(criterion)` which would be
used to add a new criterion. Producing the longer
syntax `dag.criteria.criteria << my_new_criterion` vs. `my_dag.criteria
<< my_new_criterion`.
Merge pull request #68 from erran-r7/criteria-concat-fix

Add `Criteria#<<(criterion)` to stop repetition
Bump version for minor release.
Clean-up of comments on credentials.
Updating comments.

Honestly, just testing that push will work correctly from new computer.
add support for new <Description> tag in AssetGroupConfig
add support for new <Description> tag in SiteConfig
use conditional to match original behavior on description
get the description text and not the element!
refactor group to_xml to use as_xml like sites
Make sure that we don't use a nil description in site or group saves
fix a typo in dynamic - line 50
Merge pull request #73 from vidkun/master

fix a typo in dynamic - line 50
Merge pull request #70 from gschneider-r7/master

Add support for new Description tag in site and group configs