add cultured mouse & human fibroblast dataset

[neuromusic]

For demos and tutorials, we often use an ISS dataset with cultured cells, but this dataset is not available in starfish.data. We should provide API access to this dataset so we can use it for documentation and users can have a dataset with ground-truth indicators of cell types.

@dganguli 's description of these data...

data correspond to sequening 4 bases of the beta-actin gene in co-cultured mouse and human fibroblasts. these 4 bases only differ in one position, a SNP between species. as such, these data offer ground truth information to validate a cell typing exercise

Path to spacetx experiment file: https://d2z4zivcmlmaj1.cloudfront.net/ISS/mouse_vs_human/experiment.json

Citation: https://www.ncbi.nlm.nih.gov/pubmed/23852452

[neuromusic]

closed with #1741