Changing default raiser strategy to SignalRaiser.

This will allow for rescuing Exception without breaking expected Unix signal behavior (ex. SIGINT).
Changing where strategies are initialized.

This should allow for easier creation of external strategies, since
initialization (and passing parameters, validation, etc.) now happens
outside of the gem.

Other changes:
* improving log formatting and alignment
* renaming InvalidResult to ResultInvalid (to be more consistent with
the naming in other libraries)