New Streams

* Multiple filters per stream
* It's now possible to append data to compressed streams
* Stream compression is reentrant (multiple calls to `compress!` will have
  stream deflated only once).
* A few additional filters from PDF spec
  * ASCIIHex
  * ASCII85
* DCT filter is a stub as it's used only in JPEG images. It doesn't
  decode/encode JPEG images.
Fixed Stream on Ruby 1.8
Removed unused filters

Threre probably should be a gem with all the filters that are not used in
the core.
Merge branch 'streams' of git://github.com/cheba/prawn