Change 'zcov' to be a multi-tool.
 - 'zcov-\(*\)' is now 'zcov \1'.
Add a basic functionality test.
[genhtml] Rip out code for augmenting reports with KLEE coverage info.
 - This was too KLEE specific (and unused) to be worth keeping around.
[genhtml] Use CSS border-radius instead of generating rounded corner images.
[genhtml] Use more modern HTML to draw the percent-covered bar.
 - This drops the last dependency on PIL.