Platform.Value is causing I/O -> retain value instead

Platform.Value is calling Directory.Exists -> retain value instead
(there's no need to cause I/O on every call)
suggested changes

- made 'Value' property readonly field
- moved property IsUnix further up
made enum FinePlatformID public

fix for:

././Platform.cs(38,41): error CS0052: Inconsistent accessibility: field
type `Mono.WebServer.FinePlatformID' is less accessible than field
`Mono.WebServer.Platform.Value'
././FinePlatformID.cs(30,7): (Location of the symbol related to previous
error)
Compilation failed: 1 error(s), 0 warnings
Merge pull request #66 from hanswolff/master

Platform.Value is causing I/O -> retain value instead