feat(urls): support relative CSS / template URLs in components

Example:
When defining a component called 'foo' in file
'/myproject/has/many/folders/foo.dart', prior to the change you would
do so thusly:
@Component(
   selector: 'foo',
   templateUrl: '/myproject/has/many/folders/foo.html',
   cssUrl: '/myproject/has/many/folders/foo.css'
)

The above is still valid, but now can also be defined rather more
succinctly as:
@Component(
   selector: 'foo',
   templateUrl: 'foo.html',
   cssUrl: 'foo.css'
)

A full table of what URLs will be changed/unchanged is below:
You Say:				Transformer replies:
'packages/foo/bar.html'			'packages/foo/bar.html'
'foo.html'				'packages/foo/bar.html'
'./foo.html'				'packages/foo/bar.html'
'/foo.html'				'/foo.html'
'http://google.com/foo.html'		'http://google.com/foo.html'

*Note  that as  shown any absolute paths you define will not be
bothered by the transformer, so if you have a templateUrl living at
<root>/foo.html, you can still reach it with '/foo.html'!