* moved bytes_per_sector and total_sectors from FormatVolumeOptions::new
to a dedicated methods (it is optional now)
* calculate total number of sectors using seek() to disk end if not provided
* added more doc comments
* added assertions about disk handle position being zero
Previous implementation did not zero allocated cluster so created directory
could have garbage contents.
Bug exists in code since version 0.2 and all code using create_dir is
affected.
This commit also improves tests of volume formatting so a dirty partition
is used allowing to detect bugs like this.
