Move SerilogLogMessageFormatter to correct namespace.

This should have been made in #292 but we missed that.
`SerilogLogMessageFormatter` is in the old namespace.

This should be backwards compatible, sort of . As long as no
``` C#
using Akka.Logger.Serilog;
```
statement exists in files where `SerilogLogMessageFormatter`  is used everything should compile (with a warning that `SerilogLogMessageFormatter` in the old namespace is obsolete).

If a both namespacese are used in a file where `SerilogLogMessageFormatter` is used
``` C#
using Akka.Logger.Serilog;
using Akka.Serilog.Event.Serilog;
```
... the user will get a compile error. The resolution is to remove the second using statement.