Moved Context and ContextHelper to javaposse.jobdsl.dsl

To remove distinction between top-level contexts and deeper nested contexts, I changed the top-level context classes (`Job`, `View`, `Folder`, `ConfigFile`) to implement the `Context` interface.

Since `Context` and `ContextHelper` are now used from top-level DSL classes, I moved them from the `helpers` package to the top-level package.