Add support for modifying relations based on object metadata

[acannistra]

Adding activity classification to the pre-import pipeline (https://github.com/trailbehind/USFSConflation/pull/32) has revealed that we will sometimes need to ensure that new OSM objects added during conflation are also added to relations.

This PR implements the ability to include newly-created OSM objects into existing Relations. We do this by looking for a special Tag name prefix (currently _member_of_). These tags should contain as their Values the ID of a relation that the objects should be added to.

After creating a mapping from ID -> Relation, we lookup the relations referenced in the _member_of Tags and add the OSM object that has that tag to the corresponding relation in the aforementioned mapping. We also keep track of the modified relations.

Once all objects are processed, we output a list of modified Relation objects and create a <modify> tag for them in the changefile.

This feature is disabled by default and can be enabled with the --modify_relations flag.

I haven't tested this fully yet.

[JesseCrocker]

Looked over code, LGTM

[acannistra]

We're not going to do this anymore. Perhaps we will merge this later on, but it's not going to be in this version.

[acannistra]

Putting some docs here in case we get back to this:

Relation Management
====================

This module provides support for modifying Relations. It supports a few use-cases. 

1. A <create> tag will be created, and some objects within it need to be 
   added to relations that already exist. In this case, <modify> tags are 
   created that modify the Relations in question. 
2. more later probably

It is important to note that this module is STATEFUL. It should probably be a class. 
If you need to clear the relations DB and modified relations list, you can use _reset().

Insertion in to Existing Relations
-----------------------------------

In order to insert an object into an existing relation a particular schema 
of the input data is required. In particular, any object that is to be inserted
into a Relation must contain a Tag with a user-specifiable Key
and a Value that represents a comma-separated list of the IDs of 
Relations that the object should be inserted into. 

The default Tag Key that is used is `_member_of`. (To use another, pass it as
the `relation_tag` argument to `get_modified_relations_for_object`). 

`modify_relations_with_object` is responsible for modifying a local
database of Relations with by objects to them, as specified in the object itself. 

Before running `modify_relations_with_object`, a local database of
Relations must be generated. This is done by providing a list of 
Relation IDs to `get_relations`. This is an expensive operation, 
so should be done as infrequently as possible (likely just once).

The list of IDs provided to `get_relations` should represent all relations
that need to be inserted into. 

After processing objects is complete, call get_modified_relations 
to obtain a list of modified relations that can then be used to create
<modify> tags in a changefile.