- All the `get_*` and `put_*` methods that take `T: ByteOrder` have
  a `where Self: Sized` bound added, so that they are only usable from
  sized types. It was impossible to make `Buf` or `BufMut` into trait
  objects before, so this change doesn't break anyone.
- Add `get_n_be`/`get_n_le`/`put_n_be`/`put_n_le` methods that can be
  used on trait objects.
- Deprecate the export of `ByteOrder` and methods generic on it.

Fixes #163 

This commit is a significant overhaul of the library in an effort to head
towards a stable API. The rope implementation as well as a number of buffer
implementations have been removed from the library and will live at
https://github.com/carllerche/bytes-more while they incubate.

**Bytes / BytesMut**

`Bytes` is now an atomic ref counted byte slice. As it is contigous, it offers
a richer API than before.

`BytesMut` is a mutable variant. It is safe by ensuring that it is the only
handle to a given byte slice.

**AppendBuf -> ByteBuf**

`AppendBuf` has been replaced by `ByteBuf`. The API is not identical, but is
close enough to be considered a suitable replacement.

**Removed types**

The following types have been removed in favor of living in bytes-more

* RingBuf
* BlockBuf
* `Bytes` as a rope implementation
* ReadExt
* WriteExt