Update docs regarding Popper

 * Popper is returned, as of commit e084a1b

 * A Popper must be exhausted before subsequent operations.
   Otherwise, the source string will be abandoned,
   and operations that don't change next_in (namely, finishDeflate
   and flushDeflate) will access memory that might have been
   garbage collected.
Merge pull request #8 from joeyadams/popper-doc-fixes

Update docs regarding Popper (not a callback function argument anymore)