utils: draft implementation for queue drop tracer
Issue #27    Fix hop counting bug when NACKs are enabled
Refs #25   Merge branch 'issue-25-rtt-estimation'
Refs #27   Merge branch 'issue-27-hop-count-fix'
src: Creating file to track current version of ndnSIM (in addition to git tags)

+ AUTHORS file
examples: Adding new example to illustrate use of L2Tracer to trace aggregate packet drop rate on nodes
docs: Documenting new L2Tracer with a scenario and graph building example
Merge feature branch 'feature-l2-tracer'
src: Update AUTHORS file
model+cs: Now it is possible to get pointer to ContentStore from cs::Entry

In some cases it is desirable to know which ContentStore (=on which
Node) a particular cs::Entry belongs to.  Now it is possible to
accomplish this task using cs::Entry::GetContentStore method.
docs: Adding google mirrors and new section referring to the recommended way to run custom simulation using the provided template