Migrate to hammer-cli

## Need

```gherkin
As a developer
I want to migrate the Foregit library as a hammer-cli plugin
So that I can create a standard tool between Foreman and Git
Which is easy to use and integrate 
```

## Deliverables

- A hammer-cli plugin, which has the `pull` and `push` command to sync Foreman and Git
- A configurable plugin 

## Solution

#### Prerequisites
- [hammer-cli](https://github.com/theforeman/hammer-cli/blob/master/doc/writing_a_plugin.md)
- First try https://github.com/marianitadn/foregit/pull/12/files
- Plugin example https://github.com/theforeman/hammer-cli-foreman

#### TODO
- [x] Add hammer-cli as dependency
- [x] Rename library  - hammer_foregit
- [x] Remove settings configuration, use the one provided by hammer
- [x] Update commands, use the hammer Command class to add new commands
- [x] Fix https://github.com/marianitadn/foregit/issues/14#issuecomment-49555912
- [x] Fix tests