Refactor the deflate, gz and zlib modules & rename types

[sbstp]

I've re-factored the modules and types according to what was suggested in #75. Using pub(crate) in a few places, I was able to avoid having to create multiple layers of modules. Here's a summary of the changes:

• Split deflate.rs, gz.rs and zlib.rs into multiple files. Each module has a mod.rs, bufread.rs, read.rs and write.rs.
• The structs were renamed properly. EncoderWriter becomes write::(Deflate|Gz|Zlib)Encoder. DecoderReader becomes read::(Deflate|Gz|Zlib)Decoder. This was done for all the combinations.
• The *Buf structs are moved to their respective bufread submodule.
• The *Reader structs are moved to their respective read submodule.
• The *Writer structs are moved to their respective write submodule.
• Sprinkled a few pub(crates) here and there to keep the API unchanged.
• Grouped the struct declarations with their impl blocks in each file.
• The unit tests were moved to their respective mod.rs.

[sbstp]

I can see in the travis log that I forgot to cleanup the imports when different features are used. On it.

[alexcrichton]

Thanks so much for this! And wow that's quite the diff :)

I'll poke around with this and probably merge promptly after!