Access to form inputs

This was branched from my spec corrections branch.

I wanted to have access to both valid and invalid inputs without having to monkey patch the Report class. I have introduced a `value` method to the Report.

```ruby
messages = FormKeeper::Messages.from_file #...
report = FormKeeper::Report.new(messages)
record1 = FormKeeper::Record.new(:name)
report << record1
report.value(:name) # => foo
```

~~It will raise an ArgumentError if the name isn't found in either the valid or invalid fields.~~ To support GET requests it will return `nil` if the field can't be found.

@lyokato thoughts?