Implement better default behavior for messy time data

[adamrauh]

Rather than throw an error for non-consecutive time data, the package should use some default behavior to make a reasonable guess. This will likely consist of converting the data to factors, then integers and throwing a warning to users.

See #113 as example

[adamrauh]

Fixed in 059e5a1ade42397d55af3e90e31be7d71251f5bc