Add field converters to import task
Add field converters to import task

The CSV library can take an array of converters that convert the string
from the csv field and if any of them return a non-string value then
it uses that value. If it doesn't match any of them then it will use
the original string value.

The converters are:

    Null:    If a string matches "NULL" then convert it to nil
    Integer: If a string is all digits then convert it to an integer
    Time:    If a string matches the database timestamp format then
             convert it to a UTC time.

oskarpearson/e-petitions,telekomatrix/e-petitions,ansonK/e-petitions,unboxed/e-petitions,joelanman/e-petitions,ansonK/e-petitions,StatesOfJersey/e-petitions,StatesOfJersey/e-petitions,joelanman/e-petitions,joelanman/e-petitions,unboxed/e-petitions,oskarpearson/e-petitions,oskarpearson/e-petitions,StatesOfJersey/e-petitions,ansonK/e-petitions,alphagov/e-petitions,alphagov/e-petitions,telekomatrix/e-petitions,alphagov/e-petitions,unboxed/e-petitions,unboxed/e-petitions,StatesOfJersey/e-petitions,alphagov/e-petitions,telekomatrix/e-petitions