Ignore imported packages when building docs

[sethladd]

Just tried to build the docs for lawndart (thanks for the PR!)

See result here: http://sethladd.github.com/lawndart/

How to hide meta and metadata and only show lawndart?

[kevmoo]

addTask('docs', createDartDocTask(['lib/lawndart.dart'],
  linkApi: true,
  excludeLibs: ['meta', 'metadata']
));

...and star this issue: https://code.google.com/p/dart/issues/detail?id=5460

😄

[sethladd]

Thanks!