Data structure transformers

[kasper]

Add transformers capable of building and extracting values complex data structures like maps and lists.

Preferably these should be accompanied by support for eg. JSON creation based on these types. And insertion of maps into MongoDB.

[kasper]

Added rev. [3100] which has first draft of these transformers.

[kasper]

In addition to the currently implemented transformers in trunk, I see we also need to be able to:

• Merge maps, ie collapse two or maps into a single map
• Append lists to each other
• Extract key/value pairs from Maps using a normalized approach (each pair in a new row).
• Extract elements from lists in a denormalized approach (each element in a column, the user specifies the number of columns).

[kasper]

An additional requirement:

• Set a value on an existing map (eg. a map has been parsed, and you want to add a few key/value pairs to it).

[kasper]

Fixed.