Improve Rails error reporter and other improvements

Improve Rails error reporter

Add request parameters, "path" and "method" tags to errors reported in controllers via the Rails error reporter. (Thanks @bdewater-thatch!)

Add CPU count configuration option

Implement CPU count configuration option. Use it to override the auto-detected, cgroups-provided number of CPUs that is used to calculate CPU usage percentages.

To set it, use the the cpu_count configuration option or the APPSIGNAL_CPU_COUNT environment variable.