Add FieldIndexer

[Hendy]

consumer should be able to return a collection of fields, where each can be marked as a String, Date or Number ?

all fields prefixed with Look_ to ensure uniqueness (like the tag groups)

LookMatch object should return these (strongly typed)

[Hendy]

can already use tags for this behaviour when equality checking - this could be used for comparison checking (see:#88)

FieldQuery
{
LookTag GreaterThan,
LookTag LessThan,
LookTag GreaterThanOrEqualTo
LookTag LessThanOrEqualTo
}