Add Mail.init_mail function to decouple initialization

This would allow to initialize a `Mail` class by just using a dictionary
with the required configuration, thus decoupling this from the *Flask*
`app` object.

```python
config = {
    'MAIL_SERVER',
    'MAIL_USERNAME'
    (..)
}

mail = Mail()
mail.init_mail(config)
```
Merge pull request #66 from grupotaric/initialization

Add Mail.init_mail function to decouple initialization