#68 Add a possibility to get system time for different contexts

[Anders Widén]

Add methods to the system time interface in douf to fetch the system time for different contexts.

1. A method to fetch the system time for a given context.
2. A "convenience" method to fetch the system time from the context associated with the first connection that was opened in this thread (much like how Attach works).

Note that the douf time interface just forwards the calls to an external time provider which also has to be context aware.