Add a few samples of misclassified C++ headers

Several people mentioned in #467 that they were seeing their C++ projects
erroneously showing up as containing Objective C.

I've added a file from each of the problematic repositories:

  - https://github.com/mp13on11/dwarf_mine
  - https://github.com/miguelishawt/anax
  - https://github.com/mholt/cppcsv
  - https://github.com/coder543/libcanister

They all seem to be triggering on different aspects, since adding one sample
wasn't sufficient to correctly classify the others.

The discussion in #467 makes me think that perhaps Linuist might need to take
the rest of the repository into account when classifying ambiguous files.
Expanding TestHeuristcs to include all Objective C header files
Merge pull request #844 from github/cpp-extra-samples

Cpp extra samples