Refactor the plugin framework interface

[echlebek]

This commit refactors and renames the check, handler, plugin and mutator types in an effort to add clarity and reduce implicit behaviour.

Notable changes include:

• basePlugin is now called pluginFramework and does not try to imply inheritance. Consumers no longer anonymously embed this type.
• pluginFramework exposes exported methods so that consumers don't need to rely on getting and setting unexported fields.
• GoCheck, GoMutator, and GoHandler have been renamed to Check, Mutator, and Handler, since the term "Go" does not add any useful information. Type aliases have been added to prevent breakage.
• NewGoCheck, NewGoMutator, and NewGoHandler have been renamed to NewCheck, NewMutator, and NewHandler. The existing exported function symbols are now vars that alias the renamed functions, to prevent breakage.
• Check, Mutator and Handler now have their own Execute methods so that their consumers are no longer implicitly calling methods on pluginFramework.
• Breaking change: the Plugin interface has been removed, as it was not used anywhere in the project, or in project dependencies.

Signed-off-by: Eric Chlebek eric@sensu.io

[echlebek]

@c-kruse can you take a look at this one also? Thanks!