Digitally sign assemblies when built in TeamCity

Add the build magic for assembly signing (ref: http://wiki/How_to_Set_Up_Visual_Studio_Projects_and_Solutions#Digital_Signatures) to assemblies which might be redistributed.
Make the signing task look in the right place

Because of RedGate_CreateUpdateAssemblyItem in RedGate.Build.targets, we need to explicitly say we're not obfuscating, otherwise the signing service will look in the \Obfuscated directory for input.
Merge pull request #2 from red-gate/DigitalSigning

Digitally sign assemblies when built in TeamCity