Refactored url formatting and header construction
Stubbed Add method.  Removed '?' from Format-UrlEncoded so it can be used for the body.  Renamed $fields to $responseFields
Removed validation from Invoke-SitecoreRequest - acts as raw access to the api.

Completed Add method
Ensure content type is handled correctly.

Functions for getting/setting a Sitecore item.
Fixed Test-Guid check.

Created Remove-SitecoreItem function and normalised DELETE 200 response with normal 200 response for consistency
Added paging validation.

Housekeeping: Documentation of methods