C++ linter

Add Google's C++ linter (scripts/cpplint.py) with "lint" target in Makefile (run "make lint" to run the linter on all C source/header files) and fix all of its errors for the current codebase.

If this PR is accepted, all future Caffe contributions should be required to run "make lint" with an output of "Total errors found: 0" before merging (returns status code 0 in this case).  In some cases this may involve explicit suppression of certain 'false positive' linter errors using NOLINT and NOLINT_NEXTLINE (the latter of which I added to the cpplint.py script due to the same line NOLINT almost always making the line run over 80 characters).