New generic platform

This adds a `Platform` class that may be used to deal with platforms consistently across the OS we support at Enthought:

```
from okonomiyaki.platforms import Platform

platform = Platform.from_running_system()
print(str(platform))
print(platform.name)
print(platform.os)
print(platform.family)
print(platform.arch)
print(platform.machine)
```

@sjagoe 
Why not the pre-defined constants here?