Filter by file extension for API documentation

It is an error to access the API documentation when exist file named like ".xxx_get.json.xxx" . (like vim's swap file)
```
Server running on http://localhost:3000
Server running on http://localhost:3000/_documentation/
Request: GET /_documentation/
==> work/github/node-easymock/test/mock_data/_documentation/_get.json

fs.js:427
  return binding.open(pathModule._makeLong(path), stringToFlags(flags), mode);
                 ^
Error: ENOENT, no such file or directory 'work/github/node-easymock/test/mock_data/.test1_get.json'
    at Object.fs.openSync (fs.js:427:18)
    at Object.fs.readFileSync (fs.js:284:15)
    at MockServer.readFile (node_modules/easymock/lib/easymock.js:274:17)
    at node_modules/easymock/lib/easymock.js:413:27
    at Array.map (native)
    at Function._.map._.collect (node_modules/easymock/node_modules/underscore/underscore.js:98:56)
    at node_modules/easymock/lib/easymock.js:401:17
    at next (node_modules/easymock/lib/easymock.js:360:29)
    at node_modules/easymock/lib/easymock.js:370:13
    at Object.oncomplete (fs.js:107:15)
```

Improved to filter only ".json$" API documentation target.