Changed logic to load configurations from a YAML

Setting environment variables makes all complicate.
Load from an .yml is more simple since you don't need change .bash_rc,
.bash_profile, .zshrc, or any other.
With `codeland-start install` one file is generated in
`~/codeland-starter.yml` and you can set all config in one file. To use
this file just type `codeland-starter create NAME --file=/path/to.yml`.

The section `integrations` in YAML receives one array with the services
that you want. Eg:

```
integrations:
  - heroku
  - github
```

The value is the class name of the integration.
Changes the way of Integrations verification

In ruby 1.9.3 and 2.0.0 (tested in 1.9.3, 2.0.0, 2.1.1 and 2.1.2) if I
check for `const_defined?('Bla::Foo')` I got NameError. Testing with
`const_defined?('Bla') && Bla.const_defined?('Foo')` works.
Merge pull request #1 from codelandev/yaml

Load config via YAML