Merge and internalize assemblies internal to Hangfire.Core 

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:**

* [x] Rename `Hangfire.Merged.dll` to `Hangfire.Core.dll`.
* [x] Update `nuspec` files.
* [x] Test changes.