Added support for culture-sensitive formatting.

Allow to format log events using IFormatProvider.
Logger-wide IFormatProvider support.
Merge branch 'master' of https://github.com/felixg/serilog into felixg-master
Incorporated culture-specific formatting into MessageTemplateTextFormatter, rather than Logger and LogEvent.
Dropped setting of format provider on base string writer, to maintain consistency with other parts of the code.