[MODEL] Added integration test for paginating a sorted search

Related: #258
[MODEL] Added, that SearchRequest class stores the `options` as attribute
[STORE] Changed, that search requests are executed through SearchRequest class

Extracted the `gateway.search` call into a specific class, `SearchRequest`,
primarily to allow hooking into the `SearchRequest#execute!` method in Rails instrumentation.

Updated unit tests, this change should be totally opaque to the user.

Related: #238
[RAILS] Refactored and updated the instrumentation support to allow integration with Persistence::Model

When the instrumentation/lograge components are now used with the Persistence::Model models,
Rails log prints the relevant info:

    Article Search (174.8ms) {index: "articles-development", type: "article", body: {size: 50, from: 0, sort: {created_at: "desc"}, query: {filtered: {query: {query_string: {query: "foo*", default_operator: "AND"}}, filter: {bool: {must_not: {term: {deleted: true}}}}}} ...
      Rendered articles/index.html.erb within layouts/application (57.7ms)
      Rendered articles/_search_form.html.erb (1.2ms)
    Completed 200 OK in 861ms (Views: 623.9ms | Elasticsearch: 174.8ms)

Closes #238
Release 0.1.6