Add Option to Disable Etags

I noticed that Tornado is automatically calculating etags for
images, but that didn't work well with my caching strategy.  I have
added the option to disable this.  I have it on by default since
the current version of Thumbor will auto-generate etags for you.
Merge pull request #389 from RealGeeks/etag_option

Add Option to Disable Etags
Merge pull request #386 from voxmedia/new_filters

New filters: Colorize and Saturation
Release 4.7.0
Release 4.7.1 - Sorry for the bad merge guys
add an option to override server max age configuration using an url option
Added support for If-Modified-Since & Last-Updated headers
Added configuration setting to enable feature
Merge pull request #399 from Bladrak/if-modified-header

Added support for If-Modified-Since & Last-Updated headers
Better mp4 compatibility
Dealing with odd with or height when using libx264
Merge pull request #397 from wandenberg/custom_max_age

custom max age by url
Revert "custom max age by url"
Merge pull request #400 from thumbor/revert-397-custom_max_age

Revert "custom max age by url"
Max age filter
bump to 4.8.0
Merge branch 'master' into statsd
Merge branch 'master' into voxmedia_thumbor
Merge branch 'statsd' into voxmedia_thumbor