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

the newer features it offers (dependencies, improved centralized base

command building, and standardized error code/column output).
It also adds Python 3 support. The plistlib module changed

substantially between Python 2 and 3, and we didn't have working clang

support in Review Bot 2.0 for Python 3.
Some fixes were put in place for handling clang results when

encountering a compiler error. Previously, these would just fail, as the

generated plist file wouldn't exist, and we assumed it did. Now, such

errors are turned directly into comments on a file, showing the error

output.
Along with that, line ranges are corrected, correctly commenting on the

exact number of lines that an error spans.
Unit tests were added for both simulated and actual runs of clang, using

the same datasets and expectations for both, to help catch regressions

down the road.

All unit tests pass on Python 2.7 and 3.x.