Project cleanup

[sgdc3]

• [ ] Added @NotNull annotations (jetbrains)
• [ ] Remove final statements from arguments
• [x] Include Intellij misc.xml settings file
• [x] ~~Make ConsoleLogger able to run without a Logger instance (System.out fallback)~~ (Useless, removed logger usages in utility classes instead)
• [x] Fix Markdown XML syntax in README
• [ ] Suppress irrelevant inspections
• [ ] Drop Guava usages as much as possible
• [ ] Cleanup based on inspections
• [x] Drop xAuth support and converter
• [ ] Update docs
• [x] Remove unused sample files