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

I think it might still be useful to load the default schemas into a constant.