etags: add basic support for If-Match

For an API I am working with we need to set the If-Match ETags header when making DELETE requests to objects. It seems currently restangular hardcodes ETag support for If-None-Match. I have made a crude stab at modifying this behaviour.

I found this blog post [1] that explains a bit about the difference between If-Match and If-None-Match. The latter seems used for caching, while the former is necessary for resource updates to ensure consistency. I think.

[1] http://odino.org/don-t-rape-http-if-none-match-the-412-http-status-code/