deprecate URL module

[boonious]

This PR deprecates Hui.URL struct and module for a simpler function core, by using existing Elixir structures.

Solr endpoints, headers and options can now be specified via a url string, an atomic key pointing to a configuration or a {url, headers, options} tuple where:

• url is a typical Solr endpoint that includes a request handler
• headers: a tuple list of HTTP headers
• options: a keyword list of Erlang httpc options or HTTPoison options if configured, e.g. timeout, recv_timeout, max_redirect

If HTTPoison is used, advanced HTTP options such as the use of connection pools may also be specified via options.

[coveralls]

Pull Request Test Coverage Report for Build 318

• 7 of 7 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.5%) to 98.54%

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Totals
Change from base Build 311:	0.5%
Covered Lines:	135
Relevant Lines:	137

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💛 - Coveralls