bokmann/business_time — pull request #63 by grosser (2014-01-21 18:10:56+00:00)
Title: make config threadsafe and changeable via with block

@bokmann includes all my unmerged PR's + threadsafe config + changable config via with block

```Ruby
BusinessTime::Config.with(:end_of_workday => "2pm") do
BusinessTime::Config.with(:beginning_of_workday => "1pm") do
... do some calculations ...
end
end
```

/cc @morten @aghassipour