Bump some dependencies versions (JS related)
Bump version to 0.1.13
Remove Handlebars and LESS integration
Implement plugin system:

  ZAssets will load the plugins configured in the `plugins' config key.

  Plugins must be authored as ruby gems and obey the following contract:

  * gem must be named `zassets-plugins-PLUGIN_NAME';
  * gem must implement a Registrant class, under the namespace
    ZAssets::Plugins::PLUGIN_NAME and implement #initialize(config) and
    #register methods.

  ZAssets will require each configured plugins, build an instance of the
plugin Registrant class with the current config, then call the #register
method on that instance.
Bump version to 0.2.0
Use a secure source in Gemfile
Improve git ignore file syntax
Add support for custom Gemfile
Improve gemspec:

* Add lib directory to load path only once;
* Remove extra #map call for executables files;
* Add mising test files.
Ignore tmp directory from version control
Use explicit subject in config spec
Use ruby 1.9 hash syntax
Remove useless return statement in Config
Remove useless reference to self in SprocketsEnv
Improve gemspec dependencies declarations:

* Include sass as an explicit dependency;
* Remove explicit JS runtime dependencies, and rely on execjs like
  before;
* Improve coffee-script dependency requirement.
Bump sprockets dependency to latest version
Bump puma dependency to latest version
Bump rspec dependency to latest version
Bump version to 0.2.1