#8 Add ability to free memory

[Arttu Miettinen]

When reading large files (several gigabytes), it would be useful to be able to free cached sectors when operating in read only mode. CompoundFile.Commit works only in update mode. Perhaps a method like this in CompoundFile class could do the job:

/// <summary>
/// Release all non-dirty loaded sectors.
/// </summary>
public void ReleaseData()
{
    for (int i = 0; i < sectors.Count; i++)
    {
        Sector s = sectors[i];
        if(s != null && !s.DirtyFlag)
            sectors[i].ReleaseData();
    }

    GC.Collect();
}