Add version 1.4 changelog docs.

After speaking to @diox in the #django-compressor channel on irc, I decided to contribute a changelog. I'm not sure what changes are backwards incompatible, nor how granular you would like the changelog to be, but here's a start anyway.

I generated a list of the PRs that have been merged in post-1.3 by doing the following:

```
$ pip install nodeenv
$ nodeenv -p
$ npm install -g github-changes
$ github-changes -o django-compressor -r django-compressor -a -b develop  --only-pulls --use-commit-body
```

This generated a file containing the following, which I have attempted to convert into the ``changelog.txt`` file in the docs folder:

### upcoming (1.4) (2014/04/23 23:32 +00:00)
- [#312](https://github.com/django-compressor/django-compressor/pull/312) Fixed CompilerFilter to always output Unicode strings (@anttihirvonen)
- [#382](https://github.com/django-compressor/django-compressor/pull/382) Clarifying default for COMPRESS_CACHE_BACKEND (@alecxe)
- [#406](https://github.com/django-compressor/django-compressor/pull/406) Fix #405 - compatibility with html5lib 1.0 (@ambv)
- [#402](https://github.com/django-compressor/django-compressor/pull/402) Set timestamps of files written by GzipCompressorFileStorage (@bgilbert)
- [#387](https://github.com/django-compressor/django-compressor/pull/387) Add make target for setting up test environment (@codeinthehole)
- [#439](https://github.com/django-compressor/django-compressor/pull/439) add usedevelop to tox testenv (@RonnyPfannschmidt)
- [#463](https://github.com/django-compressor/django-compressor/pull/463) Fix for extracting version number in Python 3 (@maikhoepfel)
- [#451](https://github.com/django-compressor/django-compressor/pull/451) Configure tox and requirements to test with django 1.6 (@diox)
- [#427](https://github.com/django-compressor/django-compressor/pull/427) Make CssCompressor class easier to extend (@jor123)
- [#453](https://github.com/django-compressor/django-compressor/pull/453) Support for the kind in the endblock (@WoLpH)
- [#456](https://github.com/django-compressor/django-compressor/pull/456) Allow data-uri filter to handle external/protocol-relative references (@shacker)
- [#447](https://github.com/django-compressor/django-compressor/pull/447) Fix windows line endings in offline compress (@apmorton)
- [#416](https://github.com/django-compressor/django-compressor/pull/416) Don't raise an error if BeautifulSoup isn't installed (@damianmoore)
- [#444](https://github.com/django-compressor/django-compressor/pull/444) + add rcssmin (@Mystic-Mirage)
- [#464](https://github.com/django-compressor/django-compressor/pull/464) -Fix: compare against six.string_types. (@Xaroth)
- [#437](https://github.com/django-compressor/django-compressor/pull/437) change DEBUG_TOGGLE default value from 'None' to None (@fsw)
- [#469](https://github.com/django-compressor/django-compressor/pull/469) Fix README.rst to show percentage instead of 'unknown'. (@iknite)
- [#472](https://github.com/django-compressor/django-compressor/pull/472) Try import django.utils.simplejson, except import json (@agriffis)
- [#474](https://github.com/django-compressor/django-compressor/pull/474) Fixed grammar in Readme (@vishalsodani)
- [#468](https://github.com/django-compressor/django-compressor/pull/468) Tests fail with libxml-2.9.1 (@agriffis)
- [#473](https://github.com/django-compressor/django-compressor/pull/473) Spelling fixes in README.rst (@entrepreneurj)
- [#478](https://github.com/django-compressor/django-compressor/pull/478) Fix: #477 Enhanced Error display if COMPRESS_ROOT nor STATIC_ROOT is defined (@iknite)
- [#480](https://github.com/django-compressor/django-compressor/pull/480) Fix #477: Add install instructions for COMPRESS_ROOT and STATIC_ROOT. (@iknite)
- [#494](https://github.com/django-compressor/django-compressor/pull/494) Spelling correction in docs/behind-the-scenes.txt (@eric-fish)
- [#488](https://github.com/django-compressor/django-compressor/pull/488) Offline compression for Jinja2 with Jingo and Coffin integration (@lucastan)
- [#489](https://github.com/django-compressor/django-compressor/pull/489) Document settings.COMPRESS_CACHE_KEY_FUNCTION (@adamjforster)
- [#498](https://github.com/django-compressor/django-compressor/pull/498) Relax the test for a configured CachedLoader. Fixes #493. (@mikebryant)