Upgrade version to 0.2.6 (#234)

Changes:

  - 'run_id' value automatically included in ExecutionContext context
  stack. This is a uuid.
  - Config system update:

  This is a significant change in the config system. Now the top level environment objects (and all descendants) are now part of the dagster type system. Unique types are generated on a per-pipeline basis. This unlocks a few things:

1. The entirety of yaml config files are now type-checked in the same fashion as the user-defined config.
2. One can now pass dictionaries to execute_pipeline that mimic the yaml files exactly. You no longer have to use the dagster.config APIs (although those still work)
3. The entire config system is queryable via graphql (and therefore shows up in dagit). This adds some noise to the type browser (we can mitigate that soon), but this will enable the building of a config-editor is fully aware of the dagster type system.
4. This has one *breaking* change. The yaml file's format has changed slightly.

Previously:

```
context:
   name: context_name
   config: some_config_value
```

Now:

```
context:
   context_name:
       config: some_config_value
```

BREAKING CHANGE: Config format change. See above.

python_modules/dagit/dagit/version.py

@@ -1 +1 @@
-__version__ = '0.2.5'
+__version__ = '0.2.6'

python_modules/dagster/dagster/version.py

@@ -1 +1 @@
-__version__ = '0.2.5'
+__version__ = '0.2.6'