Fix failing bug when running tasks with scope

When there is scope settings in Guardfile and run a task like `guard
list`, it fails with:

ERROR - Invalid Guardfile, original error is:
> [#] undefined method `[]=' for nil:NilClass
/path/to/guard/lib/guard/setuper.rb:98:in `block in setup_scope': undefined method `[]=' for nil:NilClass (NoMethodError)

`guard list` calls `DslDescriber#list` and it calls
`#_evaluate_guardfile` directly and then `Dsl.scope` calls
`Guard.setup_scope` but `@scope` is not initialized. This is caused by
`DslDescriber#initialize`. It doesn't call `Guard.reset_scope` while
`Guard.setup` calls `_load_guardfile` which calls `evaluate_guardfile`
after `_reset_all`.
Merge pull request #614 from yous/patch-task-with-scope

Fix failing bug when running tasks with scope