Tag feature implementation

I implemented a tag feature. This resolves #23. Please see the [tags.md](/akanevsky/tinyissue/blob/develop/tags.md) file in my branch for a full description. 

Here are some screenshots...

**Project activity feed with tags:**

![project-activity-feed-with-tags](https://f.cloud.github.com/assets/5730760/1660193/52c93ecc-5bb6-11e3-9bd4-195b82b54da7.png)

**Issue list with filter and tags:**

![open-issue-with-filter-and-tags](https://f.cloud.github.com/assets/5730760/1660195/52c9d1a2-5bb6-11e3-87ce-5d32e39df3af.png)

**Issue with tags:**

![issue-with-tags](https://f.cloud.github.com/assets/5730760/1660196/52ca6f54-5bb6-11e3-9183-c2ca7765b0e3.png)

**Tag administration homepage:**

![administration-index](https://f.cloud.github.com/assets/5730760/1660194/52c94c32-5bb6-11e3-8efd-f5cc0cbb2b30.png)

**Edit tag screen:**

![administration-edit-1](https://f.cloud.github.com/assets/5730760/1660191/52c0b8e2-5bb6-11e3-9364-013838b9adc0.png)

**Edit tag screen with color picker:**

![administration-edit-2](https://f.cloud.github.com/assets/5730760/1660192/52c82eb0-5bb6-11e3-8fa6-acad52483684.png)

Hope you enjoy, and send some feedback.