new scheme for EPA; minor refactor

Hi, this gem is great! It works like a charm and helped me a lot in a time of need. I would like to make a small contribution:

1. add EPA scheme dummy file, schema file
2. modify Paxmex::Parser#new to also accept a file path to the schema (allow other schemas)
3. modify Paxmex::Parser#parse_section to separate required and optional parameters
4. add to Paxmex::Schema#fiield a date(..) and time(..) type accepting a pattern (allow other time formats)
5. add to Paxmex::Schema#parse_decimal a simple regexp match (helps to identify incorrect field ranges)
6. remove Paxmex::Parser::SCHEMATA - it is now loaded during initialize (load only required schema)
7. add corresponding rspec tests

Where does this requirement stem from? It doesn't seem illogical that there might be more than one abstract section (though I don't believe this is the case for any of the file types), but two separate sections could conceivably both be abstract.