get asset writer from service manager
fix missing class property
Merge pull request #63 from rhysr/assetwriterfactory

get asset writer from service manager
add last modified cachebuster
added tests
add null cache buster strategy if no cache busting is desired
add docs
added "Strategy" to strategy class names
Merge pull request #64 from rhysr/cachebusterworker

Tidied up cache buster url generation
tag information
fix typo
Merge remote branch 'upstream/master'

Conflicts:
	README.md