Fix for #196 LogContext to work cross-domain

Had an issue today with cross-domain integration tests failing

Read [this](http://blog.stephencleary.com/2013/04/implicit-async-context-asynclocal.html) article a few days ago thought I'll try make serilog work cross-domains.

using a [failing test](https://github.com/serilog/serilog/issues/128#issuecomment-53998142) from #128 and test seems to pass now.