Added acceptable logLevels for grunt config
docs: Fix language.
Kill background process on process exit

Make sure that the background process is killed when the main process
exits.
chore: Rename singleRun task.
fix: Use `grunt.option` to pass `grep` as `client.args` to mocha.
fix: Actually override the browsers array as documented.

Fixes #94.
fix: Set client default args.

Fixes #79
feat: Merge all `client.args` as suggested in #79.
feat: Allow for passing of all sorts of client.args again.