Add login logic

[NikitaShkaruba]

You can start developing components right now, but server logic will be available after @Vergir will complete this issue #13

[NikitaShkaruba]

@petitemiu , commit 9369b43 , is pretty okish for building up your feature on top of it.

Quick recap about your work in a new version:

• Well, first of all, read about sessions in spring, because maybe all the session logic already exists in framework himself, and you don't have to write it manually.
• Then do start with backend: add login() logic to Controller, which just adds new SessionProcesor with session id as param, add sessionId to client's cookie. Test backend with curl and printlns to console.
• Build login form with react components.
• Bind backend with frontend.