Add single Resource regeneration OnDocFormSave

Also add regenerate_useragent property to allow configuration of User-Agent header sent by curl to trigger regeneration of the static files
Remove static file when Resource is deleted/unpublished

Resolves #15
Add new events for previous commit

Part of resolution for #15
Pull some common logic out into a library class
Add use_url_scheme and use_http_host properties

This allows use of these in the web server rewrite rules so you can support multi-context configurations where contexts represent different domains or sub-domains.
Resolves #5
Limit statcache to Resources with specified context keys

Add contexts property, which if non-empty, limits actions to the contexts specified. Accepts a comma-delimited list of context_keys.

Resolves #6
Bump version for v1.4.0 release
statcache v1.4.0-pl