Switch from blanket to istanbul for code coverage

By switching to istanbul we get better and more detailed code
coverage reports, since blanket doesn't support branches (which
istanbul does).

The command `grunt coverage` still works, but outputs to the path:
coverage/lcov-report/index.html
Gruntfile: Enable mocha to run from the local install

The grunt commands used to assume mocha was globally installed.
That might not be the case, so the grunt commands should run from
node_modules.
Gruntfile: Simplify some grunt execs
Bump the code coverage to 100% (including branch coverage)

Since the switch to Istanbul instead of Blanket, there is more
detailed code coverage with support for branch checking. As this
was not done before, the code coverage dropped from the usual 100%
to slightly less. Not a big problem, but slight changes to code or
a few more tests bumped it up to 100% again, for all types of
coverage.