model: Small extension of printing out entries of the content store
(printing more specific data for specialized versions of the content stores)
tests: Fixing issue #31 related signed/unsigned comparison warning in one of the tests
model: Add use of "extern template" to prevent duplicate instantiation/initialization of templated classes

This syntax has been tested with gcc and clang, but may not be fully
supported by other compilers.  Additional testing (and optional ifdef
blocks) is required for the final solution to the problem.
examples+docs: Adding a new example to print out instantaneous PIT sizes periodically
apps: Solving a serious bug #33 in ndn::ConsumerWindow

In certain cases, the number of in-flight packets was maintained in a
wrong way, creating inconsistencies and incorrect decision on when to
send out a new Interests.  That is, the number of in-flight
packets could be non-zero, while there were no outstanding packets at all.

Thanks to Cheng Yi for the bug discovery