Add Video search without a parent account/channel

For instance, search for the most viewed video on the whole YouTube running:

```ruby
videos = Yt::Collections::Videos.new
videos.where(order: 'viewCount').first.title #=>  "PSY - GANGNAM STYLE"
```
Merge pull request #72 from Fullscreen/yt-video-where

Add Video search without a parent account/channel
Add Yt::Collection::Videos to README
Make CHANGELOG format compatible with Gemnasium

The log shown at https://gemnasium.com/gems/yt/versions does not
support markdown code ```, so just indent it instead.

[ci skip]
Merge branch 'update-changelog-for-gemnasium'
Bump coveralls and rspec
Merge branch 'bump-gems'
Remove Gemfile.lock from gem

Gems should not have Gemfile.lock in version control, see
http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/

[ci skip]
Ignore Gemfile.lock in git

Gems should not have Gemfile.lock in version control, see
http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/

[ci skip]
Remove duplicate method `underscore_keys!`
Merge branch 'remove-duplicate-code'
Remove duplicate method `underscore_keys!`

One duplication was left after 148d470b
Merge branch 'remove-duplicate-code'
Fix Yt::Collection in documentation

It’s supposed to be Yt::Collections instead
Document more partner models in README
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