find better priority representation

As discussed in #27 colored priority levels can be a problem and the current representation is not the best. So let us find better solutions:
##### arrows

![tl-priority-arrows](https://cloud.githubusercontent.com/assets/6536990/8565172/5b0e7a6c-2553-11e5-8c70-d6b7b1efb5dc.png)
##### relative

![tl-priority-relative](https://cloud.githubusercontent.com/assets/6536990/8565174/5e16e168-2553-11e5-9d92-ff523b93595f.png)
##### blocks

![tl-priority-blocks](https://cloud.githubusercontent.com/assets/6536990/8565176/623af20c-2553-11e5-9e72-d9e09ce3aac9.png)
##### multiple blocks

![tl-priority-multipleblocks](https://cloud.githubusercontent.com/assets/6536990/8565180/676ad454-2553-11e5-9e07-c787414f2429.png)


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find better priority representation #60

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find better priority representation #60

Description

arrows

relative

blocks

multiple blocks

Metadata

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

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