Examples Working Draft

This just has the backend infrastructure and models to read in example data and handle the inheritance in a depth-first way. 

Probably spending too much effort allowing for `example-<name>.yaml` files that would have a collection and then multiple examples. The interface is pretty consistent, so good there.

The generic stuff in ``giza.inheritance`` should be reusable to replace the inheritance bits in options, steps, api tables, and so forth. 

Remaining work: 
- Get reviews (@kay-kim and  @judahschvimer)
- ~~Write rendering/rstcloth code to generate rst~~
- ~~test inheritance~~

I modified the example structure a bit to look like this: 

```
collection: "sales"
pre: |
  Consider a ``sales`` collection with the following documents:
documents:
  - '{ "_id" : 1, "item" : "abc", "price" : 10, "quantity" : 2, "date" : ISODate("2014-03-01T08:00:00Z") }'
  - '{ "_id" : 2, "item" : "jkl", "price" : 20, "quantity" : 1, "date" : ISODate("2014-03-01T09:00:00Z") }'
  - '{ "_id" : 3, "item" : "xyz", "price" : 5, "quantity" : 10, "date" : ISODate("2014-03-15T09:00:00Z") }'
  - '{ "_id" : 4, "item" : "xyz", "price" : 5, "quantity" : 20, "date" : ISODate("2014-04-04T11:21:39.736Z") }'
  - '{ "_id" : 5, "item" : "abc", "price" : 10, "quantity" : 10, "date" : ISODate("2014-04-04T21:23:13.331Z") }'
---
ref: 'base'
title: 'Name of Example'
pre: |
  Grouping the documents by the ``item`` field, the following operation
  uses the :group:`$first` accumulator to compute the first sales date for
  each item:
operation:
  - language: javascript
    code: |
       db.sales.aggregate(
          [
            { $sort: { item: 1, date: 1 } },
            {
              $group:
                {
                  _id: "$item",
                  firstSalesDate: { $first: "$date" }
                }
            }
          ]
       )
post: "The operation returns the following results:"
results:
  - '{ "_id" : "xyz", "firstSalesDate" : ISODate("2014-03-15T09:00:00Z") }'
  - '{ "_id" : "jkl", "firstSalesDate" : ISODate("2014-03-01T09:00:00Z") }'
  - '{ "_id" : "abc", "firstSalesDate" : ISODate("2014-03-01T08:00:00Z") }'
...
```
This may be personal preference, but I prefer to use os.path.isfile(src) in case there's a directory that happens to have that name (assuming I'm right that this will always be a file)