Provide parameter to override integer range on EnumField.

[bckohan]

This is needed for flag fields where users want to pack more information into the most significant bits. For instance, consider the following flag enum:

class DataRate(IntFlagProperties, s('label'), s('slug', case_fold=True)):

    DAILY  =  2**1, 'Daily',     'daily'
    HOURLY =  2**2, 'Hourly',    'hourly'
    HIGH_RATE  =  2**3, 'High Rate', 'high'

    # bits 4-16 are reserved for daily data rate in seconds

    # bits 17-27 are reserved for hourly data rate in seconds

    # bits 28-32 are reserved for high rate data rate in seconds

This flag qualifies rates into categories, but for each category a higher resolution period in seconds can be packed into the msbs. EnumField would only see the three flags and use a SmallPositiveInteger for this field. The user should be able to override this behavior with something like EnumField(DataRate, bit_length=32) to force EnumField to pick PositiveIntegerField instead.