WIP: Coordinate space values

Please note that this pull request is based on #119 (PR #119 should be reviewed first).

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Following discussion at #52 with @nicholst, @satra and @jbpoline, a first implementation of the `WorldCoordinateSystem` terms is included in this pull request. 

The current hierarchy is as follows:
![image](https://cloud.githubusercontent.com/assets/5374264/3614078/7e408b6e-0dbd-11e4-862f-a7e692953340.png)

Definitions must be re-worked. In particular, for each `CoordinateSystem`, we would need to specify (as suggested by @satra): 
 1. a set of images
 2. a normalization algorithm
 3. the previous template