mod_muc_log_http: use stores pattern search to find available dates
storagemanager: add store_exists function
mod_storage_internal: add store_exists function
mod_storage_sql: add store_exists function
datamanager: add store_exists function
mod_muc_log_http: use store_exists function from datamanager
mod_muc_log_http: day_callback uses store_exists
Merge pull request #97 from jprjr/storage-enhancements

mod_muc_log_http: use datamanager:stores() w/ 'pattern' parameter, use datamanger.store_exists()