Added code coverage generation support.

CMake Unit can now generate a coverage report of all running tests.
In order to do this, the test author should put each file they wish
to have reported as part of the coverage report in the list
COVERAGE_FILES passed to bootstrap_cmake_unit. The coverage report
only covers lines (CMake's trace mode provides us with enough
information to determine branches and function calls and support
may be added in future versions)

Set CMAKE_UNIT_LOG_COVERAGE to ON to enable "trace" logging. This
will cause relevant output of CMake's trace mode to be logged to
${CMAKE_CURRENT_BINARY_DIR}/${CMAKE_PROJECT_NAME}.trace.

This file format isn't particularly useful, so a tool to convert
this data into Linux Test Project Coverage (LCov) is available by
running CMakeTraceToLCov with the following variables specified:
 - TRACEFILE: Path to a tracefile generated by CMAKE_UNIT_LOG_COVERAGE
 - LCOV_OUTPUT: Path to lcov output file.