Vlen read support

I have added my old patch for VLEN reading support which was made originally for 2.0.1 and rebased to current master. Please review that changes, I'll be happy to provide details if something is unclear. As I said in my old post it is not particularly clean because I tried to reduce the number of changes to minimum. This patch introduces circular dependency between _conv and h5t modules, I had to do some hacking to delay import of _conv inside h5t until run time, otherwise import does not work. There is a chance to improve that piece but it might need some refactoring which I was not comfortable to do.