common/Throttle.{cc,h} unit tests

![throttle](https://f.cloud.github.com/assets/433594/81101/c1219e78-62f6-11e2-8cb7-c91ef80f6289.png)
Create a new test file covering 100% of src/Throttle.{cc,h} lines of code.
The following methods are tested:
* Throttle::Throttle with and without a maximum
* Throttle::~Throttle when each pending Cond is deleted
* Throttle::take
* Throttle::get when updating the maximum ( lower or higher ),
  when going to sleep waiting for the count to lower under
  the maximum, when going to sleep because another thread is
  already asleep waiting
* Throttle::get_or_fail when there is no maximum,
  when requesting a count that is larger than the maximum, either
  when the current value is under the maximum or above the maximum.
* Throttle::wait when used to reset the maximum and wake up
  another thread asleep waiting

All asserts checking the arguments sanity are exercised ( negative argument
for Throttle::take etc. ).

Adds the LGPLv2+ licensing terms to COPYING along with the others.

Adds a Contributors section to the AUTHORS file.

Notes:
Testing asserts outputs verbose error messages that should be silenced
but it does not seem possible.

Signed-off-by: Loic Dachary <loic@dachary.org>