Split utils and sqlalchemy modules into sub-packages

[jace]

The utils and sqlalchemy modules are both rather large. sqlalchemy already has splinter modules in roles (#109) and annotations (#138). These should be reorganised, with the main namespace importing everything from below:

coaster/
├── utils/
│   ├── __init__.py
│   └── misc.py
├── sqlalchemy/
│   ├── __init__.py
│   ├── mixins.py
│   ├── columns.py
│   ├── functions.py
│   ├── roles.py
│   ├── annotations.py
│   └── immutable.py
└── db.py