Report disk capacity on Mac OS X.

- Add RSpec
- Fix OS detection for OS X (target OS usually starts with 'darwin').
- Don't parse /proc/diskstats on Mac OS X.
Extract method ServerMetrics::Disk#disk_stats

- Add specs and remove now obsolete specs.
- Refactor @disk_stats into method with build-in error handling for OS X.
- Bump version.
Merge pull request #13 from bjoernalbers/master

Fix disk capacity reporting for Mac OS X