New kcov-merge command

Hi Simon,

This pull request adds a new 'kcov-merge' command, which can be used to merge code coverate from one or more 'kcov' output directories.  It works by merging line hit data from the various cobertura.xml files generated by kcov.  It then generates an HTML report similar to what kcov does (minus the line-by-line coverage details for each file), as well as a cobertura.xml file with the merged information.

The use case for this is pulling together an aggregate coverage report from individual test suite results on multiple machines.

Regards,
    Chris