Add Before() and After() methods to Date type.

[pnawalramka]

Adds helpful utility methods Before() and After() to Date type. They are meant to provide similar functionality as standard library's time.Before and time.After methods.

Added tests!

[mpoindexter]

Do Before and After really need to operate on time.Times? I feel like you could do a lexical comparison given the date format and this would mean that you could compare without returning an error object because you can skip the time parse step.

I also don't love the addition of the Time method - logically speaking a Date encodes a calendar date, not a specific moment in time. For example, once you convert it to a Time, it's now a fixed instant, that may actually be different dates in different timezones. This is not wrong, but I've found that converting between instants and calendar dates to be a good source of errors, so if this is not needed except for the Before and After methods I would leave it out given the above comment.

[pnawalramka]

Good point regarding dependency on time.Time. I converted the methods to perform lexical comparison. The error condition still holds though, because there is no format validation on instantiation.

Regarding the Time() method, I found that most users of the library end up converting the Date instance to time.Time anyway. That said, I guess the motivation is to use functionalities similar to those being added in this PR. I think it's reasonable to not make assumptions around timezone and location and leave it up to the caller to make necessary conversions. Removed the method from this PR.