Return bytes, instead of 512 blocks (elastic/topbeat#11)

The size of the block depends on the operating system. For example, in GNU, 1K block means 1024 bytes.
Under OSX, there are 512 blocks. So, returning bytes seems to be the best solution.

sigar_unix.go

@@ -13,11 +13,9 @@ func (self *FileSystemUsage) Get(path string) error {
 		return err
 	}
 
-	bsize := stat.Bsize / 512
-
-	self.Total = (uint64(stat.Blocks) * uint64(bsize)) >> 1
-	self.Free = (uint64(stat.Bfree) * uint64(bsize)) >> 1
-	self.Avail = (uint64(stat.Bavail) * uint64(bsize)) >> 1
+	self.Total = uint64(stat.Blocks) * uint64(stat.Bsize)
+	self.Free = uint64(stat.Bfree) * uint64(stat.Bsize)
+	self.Avail = uint64(stat.Bavail) * uint64(stat.Bsize)
 	self.Used = self.Total - self.Free
 	self.Files = stat.Files
 	self.FreeFiles = stat.Ffree