Fix Unregister Bug & Add Functionality to Check Registered Type Converters

[jblacker]

The most important part of this PR is the fix for unregisterTypeConversion(Conversion<?>) where either a NPE would be thrown if a Conversion#getTypeKeys returned null, or it simply would not unregister the conversion if the instance passed is not the same instance as what was originally registered.

Other changes were minor:

• Relocated unregisterTypeConversion(Object) to be next to its overload
• Added a method to get a copy of the currently registered type conversions as an immutable map (UnmodifiableMap wrapping a new LinkedHashMap to be precise)
• Updated .gitignore file to ignore files generated by IntelliJ for it's IDE users (such as myself)

All unit tests pass.