adding tests for ipv6 addresses on URIs
handling IPv6 addresses on URIs

* very naïve and permissive regex to check for IPv6 addreses.
more tests for IPv6
No longer trim by default when splitting

Further discussion here:

  https://groups.google.com/forum/#!topic/elixir-lang-core/66gb3J0bnUA
Return error on unsupported options in Regex.compile
parsing embedded IPv4 addresses
Merge pull request #1669 from brunoro/ipv6-uri

URI.parse supports IPv6 addresses
Merge pull request #1672 from meh/regex-compile

Return error on unsupported options in Regex.compile
Merge pull request #1671 from vanstee/string-split-trim-default

No longer trim by default when splitting
Support negative indicies in Enum.at/fetch/fetch!
Merge pull request #1675 from ericmj/enum-at-fetch-neg

Support negative indicies in Enum.at/fetch/fetch!
Only support iolists on IO
Update CHANGELOG