Add fs.getCapacity and fs.attributes (#365)

- fs.getCapacity just returns the capacity of the current drive, if
   available. This will be nil on rom mounts.
 - fs.attributes returns an lfs like table of various file attributes.
   Currently, this contains:
    - access, modification, created: When this file was last accessed,
      modified and created. Time is measured in milliseconds since the
      epoch, same as os.epoch("utc") and what is accepted by os.date.
    - size: Same as fs.getSize
    - isDir: same as fs.isDir

Closes #262

src/test/java/dan200/computercraft/core/ComputerTestDelegate.java

@@ -79,7 +79,7 @@ public class ComputerTestDelegate
         ComputerCraft.log = LogManager.getLogger( ComputerCraft.MOD_ID );
 
         Terminal term = new Terminal( 78, 20 );
-        IWritableMount mount = new FileMount( new File( "test-files/mount" ), Long.MAX_VALUE );
+        IWritableMount mount = new FileMount( new File( "test-files/mount" ), 10_000_000 );
 
         // Remove any existing files
         List<String> children = new ArrayList<>();