Lazy loading: plugins are required only on first use.
Lazy mode enabled by default.
Unit tests rewritten.

The fact that gulp-load-plugins works only with plugins which export a
single function is now assumed.

Unit tests for lazy loading has been added.
Adding myself as a contributor.
Simplifies the tests by not using `extend()`.
Adds documentation about the lazy mode in the README.
Merge pull request #12 from julien-f/lazy-loading

Lazy loading: plugins are required only on first use.