* app.instrument
* app._callTracers
* res.trace

* allow to add tracer to the app level
* activate tracer from the response object

_callTrackers runs a for loop on the application tracer list. Because tracing
should impact performances as least as possible, this commit optimizes this for
loop.

Opitimization performed:

* Usage of traditional for loop instead of forEach
* Caching of tracer array length that is used to test the end of the loop.

Here are the benchmarks that inspired these optimizations:

* http://jsperf.com/array-foreach-vs-for-loop
* http://jsperf.com/for-vs-foreacay

Tracers instrumented at application level now handles a single
option object as argument.

The operation is performed when the subapp is mounted on the parent app.
A tracer that calls the parent tracers is added to the sub app.

Supertest lib provides a method called expect to test the result of a request.
This method requires a callback that takes an error object as first parametr.
If this error object is not inspected, the validation performed by expect are
not checked. Tracing tests didn't do that checking.

This commit add checking to trace tests to ensure that no error is returned
when expect method is called.

Private methods do not need to be tested