Adding option to concatenate files before processing

[jannon]

Setting the option group_first=true in the compress template tag (requires #285) will cause files of the same type to be concatenated before being handed to precompilers/filters. This will allow people to do standard code decomposition with things like .less files (having a mixins file separate from page-specific files)

For example:

{% compress css group_first=true %}
    <link rel="stylesheet" href="/media/css/mixins.less" type="text/less" />
    <link rel="stylesheet" href="/media/css/home.less" type="text/less" />
    <link rel="stylesheet" href="/media/css/foo.css" type="text/css" />
{% endcompress %}

Will concatenate the .less files so the less precompiler will properly process all the files
This provides a fix for #30

[travisbot]

This pull request fails (merged cb428ee into 3723a67).

[jannon]

okay...it looks like there hasn't been a successful build since flake8 released 1.4 with pep8 1.2 compatibility. Apparently it's got some extra conventions that the existing django_compressor codebase was not prepared for