Add MockFileSource and MockView for testing the ResourceLoader

- MockFileSource
  Will load pre-defined resources from the filesystem. It supports
  passing a string, that when it matches an URL, will simulate a failure.

- MockView
  It is just a no-op to not get in the way when testing
  resource loading.

Few more additions:

- style.json
  Specially crafted style that will look always for the same tile,
  glyph range, etc at the "test/fixtures/resources/" path. No matter
  where you look (or at what zoom), you always get the same stuff.

- vector.pbf
  This taken from a part of Taipei that will request several glyph
  ranges.

test/resources/mock_file_source.cpp

@@ -0,0 +1,47 @@
+#include "mock_file_source.hpp"
+
+#include <mbgl/storage/request.hpp>
+#include <mbgl/util/io.hpp>
+#include <mbgl/util/thread.hpp>
+
+namespace mbgl {
+
+class MockFileSource::Impl {
+public:
+    Impl(uv_loop_t*, const std::string& matchFail) : matchFail_(matchFail) {}
+
+    void handleRequest(Request* req) const;
+
+private:
+    std::string matchFail_;
+};
+
+void MockFileSource::Impl::handleRequest(Request* req) const {
+    const Resource& resource = req->resource;
+
+    std::shared_ptr<Response> response = std::make_shared<Response>();
+    if (matchFail_.empty() || resource.url.find(matchFail_) == std::string::npos) {
+        response->status = Response::Status::Successful;
+        response->data = util::read_file(resource.url.c_str());
+    } else {
+        response->message = "Failed by the test case";
+    }
+
+    req->notify(response);
+}
+
+MockFileSource::MockFileSource(const std::string& matchFail)
+    : thread_(util::make_unique<util::Thread<Impl>>("FileSource", util::ThreadPriority::Low, matchFail)) {
+}
+
+Request* MockFileSource::request(const Resource& resource, uv_loop_t* loop, Callback callback) {
+    Request* req = new Request(resource, loop, std::move(callback));
+    thread_->invoke(&Impl::handleRequest, req);
+
+    return req;
+}
+
+void MockFileSource::cancel(Request*) {
+}
+
+}