#41 Allow index.php call with just month/year

[Mark Wallis (aka serialmonkey)]

We should allow the index.php to be called using just a month/year value
such that the mod_postbydate module can correctly show the months
posts.

This RFE is 2 fold.

1. Enhance index.php to allow month specification

2. Enhance mod_postbydate to call the month correctly.