Date Based File Archiving

I made a few minor changes that allows archiving based on dates instead of rolling/sequential numbering.
The changes have minimal impact and work with the existing code/parameters as much as possible.
I found many posts/bug reports about deleting archive files with dates in the file name as I was having the same issue.
I've been testing this for a couple days and is working as expected.

Sample output showing archive files with datetime and maxArchiveFiles of 2 (The log file for 4:29 has been deleted):
![image](https://f.cloud.github.com/assets/4999672/791657/53af56e0-eb4b-11e2-94d1-8df8d2546580.png)

Config used to achieve the results in the previous screenshot (archiveDateFormat is optional and will be generated based on archiveEvery setting if not specified):
![image](https://f.cloud.github.com/assets/4999672/791667/876dd5ba-eb4b-11e2-82f2-ef8bebd6555e.png)