Modified code to exist within the FusePump\Cli namespace

PSR-0 [1] states that classes should be namespaced as:
  \<Vendor Name>\(<Namespace>\)*<Class Name>

This commit:
 - assigns all classes to the namespace FusePump\Cli
 - modifies all references to "Exception" to be fully-qualified
 - updates the tests to reflect namespace changes

[1] https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-0.md