Escape control chars in strings

Causes issues with xml serialisation of messages between ReSharper and
the external test runner. This was already handled by xunit1, since it
used xml to serialise between AppDomains. Serialises interfaces and raw
strings now, so no longer needs to escape strings.

Fixes #10
Merge pull request #17 from xunit/xunit2-escaped-strings

Escape control chars in strings. /cc #1