Documentation bug fix: Return value is Rational or Float.

 * Bio::Sequence::NA#gc_content, at_content, gc_skew, at_skew: Return value
   is Rational or Float in recent versions of Ruby. Documentation added for
   the treatment of "u" and to return 0.0 if there are no bases.
   Reported by ctSkennerton (https://github.com/bioruby/bioruby/issues/73 ).
Prevent to repeat calculations of total bases

 * Bio::Sequence::NA#gc_content, at_content, gc_skew, at_skew: Prevent to
   repeat calculations of total bases.
Doc bug fix: wrong sample code
Update lib/bio/io/fetch.rb

Use EBI's dbfetch endpoint as default since BioRuby's endpoint has been disabled for a while now.
Updated URL of EMBL-EBI Dbfetch
removed unused variables
Incompatible change: Default server in Bio::Fetch.new is deperecated

 * Incompatible change: Default server URL in Bio::Fetch.new is deprecated.
   Users must explicitly specify the URL. Alternatively, users must change
   their code to use Bio::Fetch::EBI.

 * New class Bio::Fetch::EBI, EBI Dbfetch client. This acts the same as
   Bio::Fetch.new(Bio::Fetch::EBI::URL) with default database name
   "ena_sequence".
Incompatibile change: remove Bio::Fetch.query; add Bio::Fetch::EBI.query

 * Incompatible change: remove a class method Bio::Fetch.query because
   default server URL in Bio::Fetch is deprecated.

 * New class method: Bio::Fetch::EBI.query. This can be used as an
   alternative method of deprecated Bio::Fetch.query method.
update RDoc documents for Bio::Fetch
Issue about Bio::Fetch and BioRuby BioFetch server is resolved

 * Issue about Bio::Fetch is resolved by recent commits.

 * The BioRuby BioFetch server is deprecated. There is no hope to restart
   the service again. EBI Dbfetch server is an alternative.
documentation about incompatible changes of Bio::Fetch