Fix Knife command category

Set the  category explicitly to "digital_ocean", as by default Knife
uses only the first part, i.e. "digital". The category is used at least
when listing subcommands, for example. `knife -h`.
Merge pull request #7 from tmatilai/knife-category

Fix Knife command category
make category DRY
prepare release
credits
add basic test for VERSION definition
add travis-ci, dependency + code climate icons
allow jruby to fail, add ruby 2.0.0

1.7+ JRuby doesn't support C-extensions which yajl-ruby, a dependency by chef, requires.
WIP: test setup
Use standard require path for the base module

I guess the reason for absolute file system paths have been weird class
loading problems caused by Knife::SubcommandLoader. IMHO the right way
to handle those is to fix Knife[1], and in the meanwhile inform users to
run `gem cleanup knife-digital_ocean` to get rid of the old gem
versions.

[1]: http://tickets.opscode.com/browse/CHEF-3255
Remove obsolete dependencies

Readline and Chef::JSONCompat are not used anywhere.
And we only need 'socket', not 'net/ssh/multi'.
Make it possible for DigitalOceanBase's includers to use Knife.deps

Later `Knife.deps` calls override earlier blocks[1], so we have to
create our own way of chaining lazy dependency loaders.

Add `DigitalOceanBase.load_deps` and inject an automatic call to it in
the inheriting class (that works only if the said class does not need
to declare `deps` itself).

[1]: http://tickets.opscode.com/browse/CHEF-3932
Move `droplet create` deps to correct class
Merge pull request #8 from tmatilai/dependency-refactoring

Dependency refactoring
Update README.md
added changelog
fix diff links in changelog