Added compiler directives to support V15 Client SDK

Added code to support a CLIENTSDKV15 compiler symbol. 

If compiling against the V15 assemblies, replace all references to the client sdk in the project with the respective V15 SDK equivalents and set the CLIENTSDKV15 compiler symbol in the project properties.

Using this directive, everything will compile normally OOTB for Office365 and for on-prem/v15 only a small change has to be made to make the assemblies compatible.

To do: update readme.md with instruction how to build the project.