Add ability to change the output path from /rcov

[TALlama]

The default remains unchanged, but the path can now be changed by setting the SIMPLECOV_RCOV_RESULTS_PATH environment variable.

In my case, I want to set it to an empty string so that the coverage report will show up in the same place in a SimpleCov-using 1.9 branch as it does in my Rcov-using 1.8 branch as we do the transition. This is important because Jenkins looks in one place of the coverage file, and the same job is processing both branches.

[ktdreyer]

I ran into this when using simplecov-rcov on Gitorious as well. Here's what we ended up doing: https://gitorious.org/capillary/capillary_rb/merge_requests/7

This functionality would be quite nice to have in simplecov-rcov. It would be even cooler if simplecov-rcov provided an equivalent "rcov" task to rake that did this.