Issue U4-5231 - Created an interface based on the LogService to replace LogHelper

Created an interface based on the LogHelper, and created a service, exposed through Application.Services. Made LogHelper obsolete.

http://issues.umbraco.org/issue/U4-5231

Assisted by @cloud46limited


We should simplify the ILoggingService interface. Anything that has a generic <T> call in it can be an extension method. We should keep this interface as simple as possible.

There's quite a few methods in the LogHelper that are there just for legacy/backwards compatibility reasons, moving forward we shouldn't have them. Any method with the 'showHttpTrace' should not exist on this interface (we can deal with that internally)