Add merge, single and mergedDir options to coverageReporter

Add mergedDir, merge and single options to converageReporter.

If merge option is set to true, the variable mergedCollector will be used instead of collectors (if single option is not set to true) and only one report will be created.
If both, single and merge, options is set to true, collectors and mergedCollector variables are used and all the reports will be created (one for each browser and one merged report).

Additionally, it's possible to specify a "merged subdir" with the mergedDir options (this works like the browser name).

Tests were added to reporter.spec that verify these features:
 - 'should merge reports' - uses the merge option and creates only one merged report.
 - 'should make and merge reports' - also uses the single and mergedDir ('all') options to create three reports (merged report is created in the '/base', dir, 'all' path).