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

See here (`lib/paxmex/parser.rb:40`):
```ruby
content.each do |section_content|
  if abstract_section
    start, final = abstract_section.type_field
    section_type = section_content[start..final]
    section = abstract_section.section_for_type(section_type)
  end
```
And here (`lib/paxmex/schema/section.rb:53`):
```ruby
def section_for_type(type)
  sections_for_types.detect { |t| t.key == type_mapping[type] }
end

def sections_for_types
  @sections_for_types ||= types.map { |k, v| self.class.new(k, v) }
end
```
It seems that we should define all vari-type sections under one abstract section. Otherwise, if the row we currently parse is a part of another abstract section, the `section_for_type` will return nil and will break the application -- that's why i decided to check for this scenario in the initialization code. I may be missing something so please correct me if I do :)