Changed 'filter-data' to accept function/array/object variables

This comes very useful when dealing with autocomplete inputs that need to pull data from the server. For instance, the following example deals with twitter's typeahead.

We define a new template:
'''javascript
$templateCache.put('ng-table/filters/typeahead.html', '<input type="text" class="sfTypeahead filter filter-typeahead" name="{{column.filterName}}" options="{highlight: true}" datasets="column.data" ng-model="params.filter()[name]">');
'''

In this case, filter.data would have had to look like this, to be able to pull stuff from the server:
'''
{
    displayKey: 'val',
    source: workplaces.ttAdapter()
}
'''

This pull request allows for that without prohibiting the usual way of doing things.