Fixes #23: Adds support for dependency injection

[bradwilson]

In order to better support dependency injection, the following changes have been made:

• Caching behavior was removed as a concern of the health check itself (and cache duration was removed from IHealthCheck)
• CachedHealthCheck is a container for health checks and their cached results, and is no longer a health check base class (directly implement `IHealthCheck instead)
• Grouping became a first-class citizen (nested groups are no longer supported)
• Partial success state for the health checks as a whole is now configured on the builder, not when you call the service
• Ability to retrieve registered health checks is now available on IHealthCheckService (HealthCheckBuilder is no longer a registered service)
• Ability to run single checks and a group of checks is now part of the public contract of IHealthCheckService (CheckExecutor has been removed)
• Timeout for the ASP.NET handler is now off of HealthCheckHandler, not GlobalHealthChecks
• Registration of health checks is streamlined for both ASP.NET and ASP.NET Core, and double-registration is prohibited

[bradwilson]

/tag @glennc @CESARDELATORRE @Eilon @davidfowl

[bradwilson]

@eilon @davidfowl Code review comments addressed.

[bradwilson]

It cost me a tiny piece of my soul with every unnecessary { and }. 😛