Allow Specification of the location of the migration directory

[dstufft]

Currently nashvegas assumes that the location of the migration directory is located alongside the file that the settings are from.

However this breaks when you use settings as a module like

settings
├── __init__.py
├── base.py
├── dev
│   ├── __init__.py
│   └── base.py
└── production
    ├── __init__.py
    ├── base.py
    └── gondor.py

This pull request adds a setting NASHVEGAS_MIGRATION_DIRECTORY that can be used to manually specify the location of the migrations directory. If this setting is missing it falls back to the previous method.

[paltman]

Cool. I think this takes care of #30 that just came in the other day.