remove deprecated fields/functions, clean up docs

[jaypipes]

Mostly documentation cleanups, but this commit does remove a number of deprecated fields and functions:

• ghw.PCIInfo.Classes (deprecated for >1 year, use pcidb instead)
• ghw.PCIInfo.Products (deprecated for >1 year, use pcidb instead)
• ghw.PCIInfo.Vendors (deprecated for >1 year, use pcidb instead)
• ghw.PCIInfo.ListDevices() (deprecated for >1 year, use ghw.PCIInfo.Devices)
• ghw.pkg/pci.Address (deprecated for >1 year, just use pkg/pci/address package)
• ghw.pkg/pci.AddressFromString() (deprecated for >1 year, just use pkg/pci/address package)

Primary documentation changes were to move the configuration options, environment-setting instructions, serialization and snapshot stuff into an "Advanced Usage" section towards the bottom of the README and focus the main "Usage" part on the domain-specific functions like CPU(), Memory() etc.

[ffromani]

it seems we need a rebase? other than that good to be merged anytime.

[jaypipes]

sorry about that @ffromani. Rebased and repushed.