Upgrades sql-query dependency to 0.0.15 (#95)

This adds support for more complex queries using and, or, not, not_and
and not_or keys.
Changes repository url to new github.io domain
Adds possibility to use "-property" to indicate a descending order in Model.find()

This avoids passing an Array like [ "property", "Z" ] to indicate ORDER
BY property DESC. This new way adds possibility to pass a String like
"-property" to indicate the same.
Changes hasMany.setAccessor to support passing an array of instances (#97)

This adds support for instance.setAssociation([ Assoc1, ... ], cb) as
well as instance.setAssociation(Assoc1, ..., cb);
Adds support for indexes on properties that are no associations (#98)

Just set index: true on the properties you want.

```js
db.define("person", {
    name: { type: "string", index: true }
});
```
Adds a new option to add multi-column indexes to models (#98)

Example:

```js
db.define("person", {
    name: String,
    surname: String
}, {
    indexes: [ "name,surname", ... ]
});
```