Add SMC Farmers' Markets data to DB

Added fields that describe farmers' markets to Organization model
Grabbed entire xls of all farmers' markets in US from data.gov
Filtered only San Mateo County markets using OpenRefine and exported it
Cleaned up the data in Google Spreadsheets and exported as csv
Wrote Ruby script to created json from farmers' markets csv
Updated rake task to load both library and markets data
Fixes #24
Merge branch 'farmers-markets'