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