Data sources

We can now access syberia data sources just like syberia models:

```R
syberia_data_sources()
# [1] "source1/source1.r" "testsource.r"
syberia_data_sources(,'test')
# [1] "source1/source1.r" "testsource.r"
```

This is assuming the following directory structure in the root of the syberia project
```
- data
  - sources
    - source1
      - source1.r
      - helpers.r
    - testsource.r
  - test
  - sources
    - source1
      - source1.r
      - helpers.r
    - testsource.r

There are no tests, but they are fully covered by `syberia_models`. Maybe I should add this in however.

@tonglu