ClassReader and BufWriter

[Maurizio Cimadamore]

Hi,
as I'm looking at the classfile PR (great work!), I have few questions 
on ClassReader and BufWriter:

* I believe they are only used for attribute reading/writing - so 
perhaps they should belong to the `attribute` package?
* BufWriter should be renamed ClassWriter (as it's the dual?)

While I understand this is in bikeshed territory, I think it would be 
nice if the number of classes in the main package was kept as low as 
possible, to let the true nature of the API (models, elements, builders) 
stand out more.

Cheers
Maurizio