Refactoring to make tart-tracing pluggable as well.

'constructConfig' can now be passed in options. Also, 'effect' is
now attached to options so that it can be accessed all the way in
tart.pluggable.
'exports' is no longer necessary.
Fixed pluggable mechanism to expose effect correctly.
Merge pull request #7 from tristanls/pluggable

Refactoring to make tart-tracing pluggable as well.