Update README.md

Remark about the AssetFactory's rootDir
Explain what workers are

I found it confusing as workers are mentioned in the "Cache Busting" section but it is not mentioned before what they are.
A bit more information about what the target path is and what CacheBustingWorker does
Reorder sections

I was at first confused what "Cache busting" and "Caching" have in common, but if I am not mistaken, they are for different purposes.

I hope this ordering makes that more clear and helps to understand what cache busting is good for.
Add a subclass of CallablesFilter that also implements the DependencyExtractor interface.

Useful for tests, among other things.
Fix totally screwed code >:-)

Better run once before push.
Fix return value if no extractor given
Merge DependencyExtractor callable into baseclass
Add a test for non-intuitive behaviour
CacheBustingWorker no longer provides two strategies and has changed the constructor's signature.

See https://github.com/kriswallsmith/assetic/pull/362#commitcomment-3208549
Add a test
Coding standards
removed unnecessary executable bits from files
Merge pull request #434 from TravisCarden/master

Remove unnecessary executable bits
removed unnecessary executable bits from files
Merge pull request #431 from mpdude/callables-dependency-extractor

Add CallablesDependencyExtractor
Merge pull request #432 from mpdude/fix-string-asset-content

StringAsset's pre-canned content not available unless load()ed
Fixed protocol-relative css imports

This fixes a mistake in the source of the CssImportFilter with protocol-relative urls. The host part of the url was lost in the process and so the import did not succeed.
Added nib on stylus filter.
Added nib on package.json.