Add a DataSource for OpenWorm backers

[mwatts15]

Current list is in c302/c302/data/adopters.txt
Sub-tasks

• Define a FileDataSource for the adopters.txt file in owmeta/sources.py
• Define a property for attaching openworm supporters -- the URI should be under http://schema.openworm.org/{release_year}/{release_month}
• Create a DataTranslator to attach the names to their corresponding cell. Look up by name
• Add a translation to save_data.py and include the result in the 'http://openworm.org/data' DataSource
• Test that you can get the backers for a few cells from the 'http://openworm.org/data' DataSource's data context

Committing the backers to the openworm/owmeta-data bundle will happen in a later when the schema is updated and new bundles are released.