This updates FBInferTool to subclass the newer BaseTool, and to use

the newer features it offers (dependencies, improved file extension

handling, centralized base command building, and new comment options).
A lot of the code is now better organized, leveraging these new tool

features. Report parsing is a bit more consistent now, with results

being sorted by filename, line number, and then column number.
Most of FBInfer is straight-forward, but when using the CMake build

type, there is an extra "compile" step that generates a Makefile and

switches the build type to "make".
If working with a multi-file build type, the command will be run in

handle_files(), and the resulting report loaded.
If working with a single-file build type, handle_file() will take

over, generating reports for each file, and combining all reports into a

single report.
In either case, the result is a report. handle_files() loops over

that, adding comments to the report.
Unit tests were added for single and multi-file builds and for the

special CMake compilation step.

All unit tests pass on Python 2 and 3.