Add additional query parameters to GetRepositoryIssues

[SeanTAllen]

`GetRepositoryIssues` currently supports `labels` and `state` query parameters. The GitHub API supports several more that would be useful:

Parameter	Type	Description
`sort`	String	`created`, `updated`, or `comments`. Default: `created`
`direction`	String	`asc` or `desc`. Default: `desc`
`since`	String	ISO 8601 timestamp. Only issues updated at or after this time
`per_page`	Integer	Results per page (1-100). Default: 30

These parameters can be added to the existing `GetRepositoryIssues.apply()` method and `Repository.get_issues()` as additional defaulted parameters, and to `_build_url()` for URL construction.

See also Discussion #54 for whether `sort` and `direction` should use union types instead of strings.