Need ability to specify which MSBuild platform to use.

[marcosnz]

For ny build scenario I have to build an "Any CPU" target on a.64bit server using the x86 version of MSBuild.
Currently cake uses the build target to determine which MSBuild version to use. In my case it chooses 64bit because the target is"Any CPU" and running on 64 bit.
Need to be able to specify x86 MSBuild in this case.

Suggested solution:
Add an MSBuildTarget property to MSBuildSettings (similar to PlatformTarget) which has three values: Auto (use default/current logic), x86 and x64. This would be passed to the MSBuildResolver.
A SetMSBuildTarget extension method will be added to MSBuildSettingsExtensions to enable fluent api.