Finishes on steps

Retain previousStep and requestedStep in customData field.
Better integration in jQuery standard workflow.
Better user experience (can go forward if form is not dirty).
Better integration of custom options.
Usage exemple :
```js
      jQuery(document).ready(function() {
	    c2cgeoform.steps.init({
	      // jQuery.steps properties
	      headerTag: 'h3',
	      bodyTag: 'section',
	      enableAllSteps: false,
	      stepsOrientation: 'vertical',
	      labels: {
	          'cancel': '${_('Cancel')}',
	          'current': '${_('current step:')}',
	          'pagination': '${_('Pagination')}',
	          'finish': '${_('Finish')}',
	          'next': '${_('Next')}',
	          'previous': '${_('Previous')}',
	          'loading': '${_('Loading ...')}'
	        },
	    
	      // c2cgeoform.steps specific properties
	      formId: '#${formid}',
	      stepsContainerId: '#steps-${formid}',
	      errorContainerId: '#${formid}_error',
	      customDataFieldId: '#${formid}_custom_data',
	      onlyValidateFieldId: '#${formid}_only_validate'
	    });
	  });
```