Added testing in JRuby for Travis.
[vcloud_director] Mocking for tasks.
[vcloud_director] strftime not iso8601 for ruby 1.8.7
[vcloud_director] Consistent usage of ensure_list! in get_disk.
[vcloud_director] Fix ruby 1.9-isms in media_tests.
Semantic Versioning (and Pessimistic Versioning Constraint) notice

I noticed some projects using Fog are setting very specific versioning constraint.  For example, https://github.com/mitchellh/vagrant-rackspace/issues/34 happened because both vagrant-aws and vagrant-rackspace are using 3 digits of precision.

The fog RELEASE.md says that Fog follows semantic versioning.  Several other projects have put up notices suggesting to use 2 digits of precision for projects that follow semver.  See https://github.com/intridea/multi_json, for example.

I added a similar notice, but softened the language because I don't think there is a clear understanding on what semantic versioning means for a project like Fog, which has multiple providers backed by services that are all evolving at their own pace.
Merge pull request #2302 from nosborn/vcloud_director_task_mocking

[vcloud_director] Mocking for tasks.
Don't load a couple MRI-only development dependencies on JRuby.
Temporary hack around #2279 to get fog loading on JRuby.

The plan is to get things largely building in JRuby and then tackling #2279 separately.
Added logging message for JRuby users to know they're at a disadvantage.
Merge pull request #2315 from nirvdrum/jruby_fixes

JRuby fixes.
Merge pull request #2312 from nirvdrum/add_jruby_to_travis

Added testing in JRuby for Travis.
[core] Make `ruby-libvirt` dependency optional

The `ruby-libvirt` dependency was commented out since it is rarely used
and a pain for developers to prepare.

The comment was removed in https://github.com/fog/fog/commit/b373e55a8f32b24c11fe4b20e773c1428df9d811
to protect JRuby but made it a requirement for MRI (& others).

This adds another guard based around the setting of `FOG_USE_LIBVIRT`
environment variable.

This hopefully will mean anyone wanting to use it can just set that
rather than editing the gemspec.

Fixes #2316
[jruby] Ignore failures until fully supported

JRuby is now being tested but does not pass yet so I'm ignoring the
failures in Travis until support is finalised.
Merge pull request #2317 from fog/remove_libvirt_dependency

[core] Make `ruby-libvirt` dependency optional
Scope the Unicode module so NameErrors make more sense.
Handle another case of the 'unicode' gem breaking things on JRuby.

This is was an omission from pull request #2315.
Merge pull request #2318 from nirvdrum/more_unicode_jruby_cleanup

More unicode jruby cleanup.
Added a JRuby workaround in a test that's failing due to a JRuby bug.
Bound the 'myns' XML namespace to keep Nokogiri on JRuby happy.

It appears JRuby's Nokogiri implementation is strict about using unbound XML namespaces.  Since a well-formed doc should have bound namespaces, I just added it here.