Updating installation docs

When following the installation docs there is a missing requirement of ceres.
Add installable wsgi file

This simplifies the graphite.wsgi configuration example.

When running graphite in a virtualenv with gunicorn, this also allows running
`gunicorn graphite.wsgi` directly, without any additional configuration.
Updating install documentation for ceres.

There currently is no formal ceres release, therefore there is no direct pip install.
It must be install via github (for the time being)
Merge remote-tracking branch 'upstream/master' into feature/wsgi
Add back the comment about preloading search index
manual revert of #538, need to rethink how to handle functions with optional deps
Merge pull request #572 from obfuscurity/revert_lowess

Revert Lowess function and numpy dependency
Fix typo for source installation of ceres in "git clone" step
bump django req for docs to match main
Merge pull request #573 from obfuscurity/master

Bump docs requirements for Django 1.4
Merge pull request #513 from brutasse/feature/wsgi

Add installable wsgi file
consistent linespacing between series functions
Merge pull request #574 from obfuscurity/series_function_whitespace

Consistent linespacing between series functions
Merge pull request #500 from justino/install_docs_2

Updating installation docs