add FeatureStore, a helper class for creating stores pre-configured with a FeatureReader

[elemoine]

This ticket suggests adding mapfish.widgets.data.FeatureStore, a helper class for creating stores pre-configured with a FeatureReader. Example of usage:

var store = new Ext.data.FeatureStore({
    fields: [
        {name: 'name', type: 'string'},
        {name: 'elevation', type: 'float'}
    ]
});
store.loadData(features);

[elemoine]

patch-Map Fish?-308-r1340-A0.diff, tests pass, please review.

[pvalsecc]

If you're sure we never need to be able to set the meta parameter, then you can commit.

[pvalsecc]

I forgot: don't we need a "require"?

[elemoine]

(In [1360]) add Feature Store?, a helper class for creating stores pre-configured with a Feature Reader?, r=pvalsecc (closes #308)