Merge and internalize internal to Hangfire.Core assemblies

This PR addresses issue #194. Following assemblies were merged with `Hangfire.Core`:

* NCrontab
* CronExpressionDescriptor
* Microsoft.Owin

All of these assemblies are used internally in Hangfire, so this change should not produce breaking changes and can be introduced in `1.x` version. Other dependencies will not be merged, since users may use their types to customize Hangfire.

**TODO:**

* [ ] Rename `Hangfire.Merged.dll` to `Hangfire.Core.dll`.
* [ ] Test the changes.