Maint: Provide error messages for all properties

Provide error messages for the username and password properties. These
messages clarify that these properties are required.
Maint: Tweak language for error message

Tweak the language for the Steam Guard code property's error message.
Maint: Return early instead of using else
Test: Add initial test framework

Add tests for SteamChatBot.Connect(). Three tests have been added to
test use cases for: when a username and password are provided, and a
sentry file does not yet exist; when a username, password, and Steam
guard code are provided, and a sentry file does not exist; and when a
username and password are provided, and a sentry file exists.
Test: Add Travis CI support
Docs: Add build status icon
Docs: Add dependency status icons