Clarifies logic for rescues in Syntax

[lorenrogers]

Rubocop provided warnings about the rescues around the
Date.parse logic. This commit adds documentation around these
functions and makes the rescue logic more specific. return nil
was added to suppress Rubocop warnings and to make the code
more explicit and easier to understand.

Closes #19

[samwho]

What exactly are the warnings? Why is what you've got here better than what was there before?

[lorenrogers]

Here's what a run of Rubocop gives me for the original code:

lib/todo-txt/syntax.rb:42:7: W: Do not suppress exceptions.
      rescue; end
      ^^^^^^^
lib/todo-txt/syntax.rb:49:7: W: Do not suppress exceptions.
      rescue; end
      ^^^^^^^
lib/todo-txt/syntax.rb:56:7: W: Do not suppress exceptions.
      rescue; end
      ^^^^^^^

This commit handles these by having the rescue clause perform an explicit action. I admit that the end result is the same, (Passing nil when the date is invalid,) but the new code/documentation makes it clear that this is the intended behaviour.

Along the way, it also removed the if line =~ CREATED_ON_PATTERN clause, making these three methods consistent and simplifying the logic a little.

[samwho]

Neato. This is the first time I've heard of Rubocop. That's an excellent condition for it to pick up on and warn you about.

You also saved us from matching on the same regex twice, which is a nice little performance win. I'll merge this now. :)

[lorenrogers]

Awesome! Yeah, I love Rubocop. Definitely helps keep code clean.

I've used Hound before, which adds in Rubocop checks by default. (More discussion: #16 (comment)) Could be worthwhile for future maintenance.