Use datapackage's base path when loading custom profiles

[vitorbaptista]

For example, consider the datapackage:

{
    "name": "test",
    "title": "Test",
    "profile": "my-data-profile.json"
}

The my-data-profile.json should be loaded relative to the datapackage.json path, otherwise we would need to use the absolute path, which isn't portable.

[roll]

If it doesn't use the data package directory for now - it's a bug.