Fix bug introduced by d58c5b2 for gem sources list

When "--list" or "-l" is passed to "gem sources", the command is
currently ignoring it.
Failing test for #672

Pull request #672 fixes `gem sources --list`, but does not include a
test to verify.

This test will verify the pull request.
Add #672 to History
Do not modify global Specification.dirs during installation.
Add #670 to History
Use RFC 2616 compatible time

RFC 2616 states that the time in the If-Modified-Since header must be in GMT.
Time#rfc2822 returns a time with time zone offset whereas Time#httpdate always
returns a time in GMT.
Add #655 to History
Be more descriptive warning when licenses is empty

Previously RubyGems said "licenses is empty" without giving any further
guidance.  This didn't help users adjust to the new warning.

Now rubygems points to the OSI license list so users have a better idea
of what to do to silence the warning.

Fixes #656
Include license list in Specification#license=

This follows #656, and gives better guidance to users setting licenses.
Improve Specification#license documentation

Now the various license methods link to #license= where the list lives.
Expand destination_dir for install location

When File.realpath is missing (ruby 1.8) RubyGems will report that a file
is being installed outside the install location when $GEM_HOME contains
"..".

Expanding the destination directory while determining the install
location avoids this false-positive.

Fixes #679
Set release version and date