Basic ESS feed parsing functionality
Added #find_coming and #find_between methods to ESS class.

They both support only standalone and recurrent date items for now,
while taking into account the "unit" and "interval" attributes into
account.
Merging fixes from 'master' into ess-parser
Implemented find_coming and find_between methods
Removed max_events parameter for find_between
Updated the README with information about parsing