Added Http request sensors entity-initializers for YAML definition

Added support for defining basic HTTP sensors at  YAML which retrieves JSON response from http polling to certain URL.

Also fixed deprecated calls on HttpValueFunctions.
`jsonPath` might be a better name, with a comment that (i think) only paths 1 level deep are supported presently.  we really should support the usual json path syntax (cf HTTP PATCH) in time!

wdyt?