Implement drafts and allow the use of time in publish dates.

This pull request adds a drafts feature and allows the use of times, e.g. 11:53 PM EST, in publish dates.

# Presentation
When any new post is created, it is a draft.

A post represents whether it is a draft or not in the posts table view. Posts now have a "draft" string in the location where the published date would be:
![screen shot 2013-09-05 at 8 54 10 am](https://f.cloud.github.com/assets/166744/1088075/68c3c192-162a-11e3-9e0b-324f222ff663.png)

A draft will only show up in local previews and will *not* be included in the generated site during publishing.

You can switch a post between draft/published by using the contextual menu for the post:

For a published post...
![screen shot 2013-09-05 at 8 54 19 am](https://f.cloud.github.com/assets/166744/1088076/6fd1d1c2-162a-11e3-9eec-978d89ac84d5.png)

And for a draft…
![screen shot 2013-09-05 at 8 54 15 am](https://f.cloud.github.com/assets/166744/1088077/7282efbe-162a-11e3-8c8c-d564f1be6976.png)

Hurrah!

# Technical Stuff
The implementation stuck to "Method B" in the [discussion](https://github.com/CarterA/Tribo/issues/16#issuecomment-23302869) for #16.

A metadata class was added so that we can manage whatever metadata goes along with a post. This data must be written to the metadata file when changed, otherwise it will not be saved.

There were no major changes per-say. Some of the changes were cleaning up the code so I could modify it (and grasp it).

## Problem
There is one problem with this pull request, albeit a small one. When you resize the window while viewing the posts view, it is possible for the title and date labels to collide.
![screen shot 2013-09-05 at 8 54 24 am](https://f.cloud.github.com/assets/166744/1088078/791feaa2-162a-11e3-9f98-5be8eb032bf7.png)

Not a huge issue, but I imagine it could be solved with some auto layout voodoo (Xcode 5?).