Use context as dict instead of kwargs in PipelineExtension

A context dict is compatible with both the jinja2 and coffin (django)
tempate interfaces. kwargs is only compatible with jinja2.

Adding 3rd party compiler section

fix for #198 including unittest

added unit test illustrating issue #198 (CSS Relative Paths with querystring/hash are not converted)

Adds documentation for cache-busting URL support with staticfiles

Stop requiring a Django version bigger than 1.4.2 since that upgrade to ...

Extending the document to contain links out to third party compilers written on top of django-pipeline. This should make simpler to find related resources and expand the django-pipeline community.

adding Template instructions for Handlebars

Fixing a grammar issue in the mustache section as well as adding settings for handlebars support

replaced closure with uglify typo

what if we use thread ?

on debug does not minify html

If settings.DEBUG is True developer does not want minifyed html. This
is needed only in production enviroment.

avoid errors for newlines in stderr

UglifyJS2 prints two newlines '\n\n' into stderr for me.
The `if stderr` in the `SubProcessCompressor.execute_command` raises a CompressionError if that happens.
I've added a `strip()` call to avoid that.