Allow datacontext to return the real values for concepts and filters

![screen shot 2014-05-29 at 11 06 52 am](https://cloud.githubusercontent.com/assets/2774472/3119075/eb405cf4-e742-11e3-936a-7edb45d6961d.png)

What is returned when a filter is applied.

This will be used to apply styles to the filter display in filter.js 

Signed-off-by: Sheik Hassan <solergiga@yahoo.com>
Did you measure the performance? In most cases attempting to optimize something incurs code complexity. I am willing to guarantee the benefit is negligible since the size of these lists will be small in the majority of cases. The benefit to optimization (it there is any at all) does not outweigh the readability of this code.

Furthermore, the underlying data type is not relevant to deciding whether to use `field.value_labels` to map. I [stated this above](https://github.com/cbmi/avocado/pull/212/files#r15725348) (a couple times). One final time.. the values should only be cleaned if the flag is flagged as enumerable or is a key-based type:

```python
if f.enumerable or f.simple_type == 'key':
    # cleaned values
```

That conditional goes below the `cleaned = None` statement and contains the if/elif/else block.