fix(template-README) Generated README now follows Plugin README conventions

Bare template of simple case from https://github.com/wearefractal/gulp/wiki/Plugin-README-Conventions. There is also a notice for developer to read related wiki topics.

Closes #3
fix(template-index): comment link for 'Writing a plugin'
fix(template): no longer generating Gulpfile or requiring gulp

Requiring a Gulpfile in a plugin is unnecessary. Vast majority of plugin authors do not use one. Tests now use gulp-util instead.
fix(README): badges and info improvements
fix(template-index): error on stream

Since stream support is optional we explicitly throw Error if stream is passed.
fix(template-test): improved testing scaffolding

* "expected" files
* test: error on stream
* test: on stream (example)

Closes #6
chore(version): Bump to v0.3.0 and updated CHANGELOG