Slack notifications

Implements methods to check if the person is late to work:
`late?(shift)`
 if he/she missed work:
`missed_the_day?(day)`
 and to check if the lunch break was too big:
`break_too_long?` 
In case of any of those happen, an slack notification will be sent to `#cagou_no_pau_no_ponto` according to the case of the event.

``` Rake check_absences:yesterday``` will run daily to check people who missed work on the day before

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* Reviewers: @thiagobrandam 
* Redmine issue: http://redmine.redealumni.com/issues/257
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