Add Fluent::Parser class for Parser plugins
Add Fluent::Formatter class for Formatter plugins
Add Parser#parse method for clear and consistent API

'call' is not clear method name for 'parsing text' behaviour.
'call' still exist for existing plugins but 'parse' is recommended from now.
Merge pull request #499 from fluent/add-filter-and-parser-classes

Add Parser and Formatter base classes