Implement functions for endpoint: Releases

[sboysel]

https://research.stlouisfed.org/docs/api/fred/#Releases

The following endpoints need to be implemented properly and consistently in fredr:

• fredr_releases() - Get all releases of economic data. (#33 )
• fredr_releases_dates()- Get release dates for all releases of economic data. (#33)
• fredr_release() - Get a release of economic data. (#33)
• fredr_release_dates() - Get release dates for a release of economic data. (#33)
• fredr_release_series() - Get the series on a release of economic data. (#33)
• fredr_release_sources() - Get the sources for a release of economic data. (#33)
• fredr_release_tags() - Get the tags for a release. (#33)
• fredr_release_related_tags() - Get the related tags for a release. (#33)
• fredr_release_tables()- Get the release tables for a given release. (#33)

The functions are to be implemented with explicit arguments for each parameter in the FRED API (see documentation for each endpoint and the discussion in #20).