Add a 'Profile' configuration and code coverage tool.

This patch adds a 'Profile' configuration to common.gypi, and when the build
is done using GCC, adds specific compiler and linker flags to cause the library
and test drivers to be instrumented for profiling (and code coverage). In addition
it adds the 'tools/gcovr' script which runs the gcov tool and gathers the output,
converting it into Cobertura-style XML output.
Convert to 'lcov' based coverage reporting.