Artifactory set and delete properties support

Added setter and deleter functions for properties.
The default value of recursive is None. In that case we accept the [Artifactory documentation](http://www.jfrog.com/confluence/display/RTF/Artifactory+REST+API#ArtifactoryRESTAPI-SetItemProperties) default:
> When a folder is used property attachment is recursive by default.

I only wanted to add 'recursive' HTTP GET parameter only if it is specified explicitly.