Bump v0.8.1

# Changelog

## 0.8.1 (2014-02-14)

### Builder
- Avoid extra mount/unmount during build. This fixes mount/unmount related errors during build.
- Add error to docker build --rm. This adds missing error handling.
- Fix regression with ADD for tar files. This stops ADD from unpacking tarballs.

### Documentation
- Download the docker client binary for Mac over https
- Update the titles of the install instructions & descriptions
* Add instructions for upgrading boot2docker
* Add port forwarding example in OS X install docs

### Remote API
* Move code specific to the API to the api package
- Fix header content type for the API. Makes all endpoints use proper content type.

### Runtime
* Do not ping the registry from the CLI. All requests to registres flow through the daemon.
- Check for nil information return in the lxc driver. This fixes panics with older lxc versions.
- Devicemapper: cleanups and fix for unmount. Fixes two problems which were causing unmount to fail intermittently.
- Devicemapper: remove directory when removing device. Directories don't get left behind when removing the device.
* Devicemapper: enable skip_block_zeroing. Improves performance by not zeroing blocks.
- Devicemapper: fix shutdown warnings. Fixes shutdown warnings concerning pool device removal.
- Ensure docker cp stream is closed properly. Fixes problems with files not being copied by `docker cp`
- Stop making `tcp://` default to `127.0.0.1:4243` and remove the default port for tcp
- Fix `--run` in `docker commit`. This makes `docker commit --run` work again.
- Fix custom bridge related options. This makes custom bridges work again.
+ Mount-bind the PTY as container console. This allows tmux/screen to run.
- Avoid extra mount/unmount during container registration. Fixes a race condition.