Post ordering by exact date

By default, posts are ordered by `YYYY-MM-DD` in the file name. This works as is intended, except for when you want to have more than one posts for a specific day. Then posts aren't exactly ordered by publish time. 
To solve this, if we introduce a `date` property to the markdown header file, we can set this `date` as the exact date for the post, and this makes correct ordering of posts possible.

a possible markdown header file:
```
---
...
date: 2014-04-30 23:02:42
...
---
```
(This is my first PR ever :D)