Client modifications

Changes:
- Moved login functions to separate LoginModule
- Created ClientInterface
- Made it possible to choose session storage directory
- Removed locale setting. All modules are based on English version of
game, so there is no point in setting other language.

New features:
- Added stil/xpath-selector to dependencies
- Now, the client returns ResponseWrapper instead of Guzzle's original
Response object. The wrapper has xpath() method, which quickly converts
response HTML body into instance of XPathSelector\Selector
Added getArticle() to PressModule
Upgraded ExchangeModule to new XPathSelector