Remove all force unwraps / force try

[stefanomondino]

After #18 is merged, we should work on SDK safety.

Code is still full of force unwraps (someVar!.something()) and force try (try! something) which is dangerous as it could lead to crashes.

We should remove all of them, apps should not crash because of our SDK.

Fixing strategy should be:

• if a method is already throwing, simply use try instead of try!. If error is not blocking, use try? and provide a fallback value
• if a method is not marked as throwing, use try? and provide fallback value with nil coalescing operator (let value = (try? something()) ?? defaultValue)
• in testSomething methods (Unit tests with XCTest, in Tests folder), simply add throws to method declaration

It's not easy to spot them all, but using Xcode looking glass and searching for try! can be a good start.