Introduced 3 new commands,

Tag
===
Builds and tags a service(s)

Example:
```
skipper tag web:v1
```

Push
====
Push a service's tag to it's registry.

Example:
```
skipper push web:v1
```

Clean
=====
Removes older local images.

Example:
```
skipper clean web
```
Merge pull request #22 from cameronmaske/tagging

Tag, push and clean.