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


@Shazwazza showHttpTrace is still important for people still using webforms, where `umbDebugShowTrace` still works just fine.