Add unit-tests#2

1. Add google tests/gmock library to project; 2. Implement interface for base classes; 3. Reimplement base classes via inheritance from class-interface; 4. Create mock objects for tested classes; 5. Implement the following test-cases: **Launching**: - [ ] Server double start (another process already running as a server); - [ ] Client is starting when a server is not running; **Transmitting**: - [ ] A client sends 1 file; - [ ] A client sends 2 files; - [ ] A client sends 3 files; - [ ] 2 clients send 2 files; - [ ] 3 clients send 3 files; - [ ] During a transmission (1 file) a server stops. The client has not to transfer not one file. - [ ] During a transmission (1 file) a client stops. The server has not received not one file. **Chunk allocation**: - [ ] Set queue size to 3 chunks and start transmitting 4 files. The server will receive only 3 files. - [ ] Set queue size to 3 chunks and start transmitting 4 files. The server will receive only 3 files. Wait for the transfer to complete. Start transmitting 3 files - all files should be transferred. **P.S.0.** A disconnect test-case can be implemented via handling a specific call of function at **N** time (**.times(50)**). For example, the client is sending a file to the server. We are able to catch a handler of processing the second chunk, kill server transfer thread and then check **_transmittedFileCounter** variable.