Promise Support and misc refactor

#115 - Add promise support to fetchr

// Before
fetcher
    .read('data_api_service')
    .params({id: ###})
    .end(function (err, data) {
        // data is returned by `data_api_service`
    });
//After
fetcher
    .read('data_api_service')
    .params({id: ###})
    .end()
    .then(function (result) {
        // result.data is returned by `data_api_service`
    });

#122 - Refactor service metadata

Metadata logic in fluxible-plugin-fetchr has been moved into fetchr itself. The usage is the same as before.

// callback
fetcher
    .read('data_api_service')
    .params({id: ###})
    .end(function (err, data, meta) {
        // meta will contain any headers returned by the `data_api_service`
    });
// promise
fetcher
    .read('data_api_service')
    .params({id: ###})
    .end()
    .then(function (result) {
        // result.meta will contain any headers returned by the `data_api_service`
    });