Add packagecloud.io provider

This PR introduces a provider for [packagecloud.io](https://packagecloud.io)

This provider allows users to push their `deb`, `dsc`, `rpm`, or `gem` build objects to their [packagecloud.io](https://packagecloud.io) package repository.

#### Options

  -  username:  the [packagecloud.io](https://packagecloud.io) user name
  -  token: the [packagecloud.io](https://packagecloud.io) API token for the username specified
  -  repository: the name of the [packagecloud.io](https://packagecloud.io) repository
  -  local-dir: the directory where the package objects to push can be found
  -  dist: the distribution string to use. The complete list of supported strings can be found on the [packagecloud.io docs](https://packagecloud.io/docs#os_distro_version)

#### Example

```
dpl --provider=packagecloud
      --username="${PACKAGECLOUD_USERNAME}"
     --token="${PACKAGECLOUD_APITOKEN}"
     --repository="repository-name"
     --local-dir="${TRAVIS_BUILD_DIR}/pkgs"
     --dist=ubuntu/precise
```


Is `mime` used anywhere?