Add `content_owner.create_asset` and Asset model

Code coverage might decrease after this commit since the
test for `content_owner.create_asset` is temporarily commented
out, given that the YouTube endpoint to delete an asset (after the test
creates it) does not work as expected.
Add `asset.ownership` to list owners of an asset

This commit adds `has_one :ownership` to the Asset model, which allows to
retrieve the Ownership of an asset.

Ownership includes different types of right owners (general, performance,
synchronization, mechanical), all of which are exposed to the asset by calling

    asset.general_owners
    asset.performance_owners
    asset.synchronization_owners
    asset.mechanical_owners

Each RightOwner includes the owner name, the ratio of ownership and the list
of included/excluded territories.
Add `update` method to Ownership

This allows to change the owners of an asset, specifying the type,
ratio and territories of each owner.
Add `obtain!` method to Ownership

This allows the authenticated content owner to take 100% ownership
of an asset in every territory.
Camelize every param unless otherwise specified

Before this commit, only LIST requests to YouTube had their parameters
camelized by default (unless :camelize_params was set to false).

After this commit, EVERY request (insert, update, delete..) has the same
behavior, since almost every YouTube API endpoint expects camel-case
parameters.

Of course, "almost" means not all of them; for instance upload a video
in chunks requires "uploadType" (camel-case) and "upload_id" (underscore)
in the same request!

Note that this version of camelize is not recursive
Use underscore, rather than camel-case params

Because of the previous commits, params can be either underscore or
camel-case and Request will transform them in the format that is
appropriate for YouTube API endpoint.

For coherence with the rest of the code, parameters are now spelled
out as underscore.
Camelize every body unless otherwise specified

Similar to the params, almost every BODY sent to YouTube API endpoints
expects camel-case attributes.

Of course, "almost" means not all of them; for instance authenticating
with YouTube requires to submit "grant_type" and not "grantType" in the body.
Use underscore, rather than camel-case body

Because of the previous commits, body attributes can be either underscore or
camel-case and Request will transform them in the format that is
appropriate for YouTube API endpoint.

For coherence with the rest of the code, body attributes are now spelled
out as underscore.