Merge and internalize internal to Hangfire.Core assemblies
Don't use ILMerge in build pipeline by default
Use `Hangfire.Core` name for merged assembly
Remove NCrontab dependency from Hangfire.Core.Tests
Exclude merged assemblies from Hangfire.Core.nuspec
Fix for disable using ILMerge by default
Conditional reference to NCrontab from tests
Remove reference to CSharp.targets from ILMerge.targets
Add conversions between AppFunc and OwinMiddleware

Automatic call to the `SignatureConversions.AddConversions` method is
not being used since internal version of `AppBuilder` class ctor is not
being called on application startup anymore.
Merge pull request #197 from HangfireIO/internalizing

Merge and internalize assemblies internal to Hangfire.Core