Composer: create from URL

New function in the composer toolbar to create a graph from a URL.
This is based entirely off of @obfuscurity Stupid Graphite Tips
https://speakerdeck.com/obfuscurity/stupid-graphite-tips?slide=12

Icon is Creative Commons:
https://www.iconfinder.com/icons/49529/link_url_icon
add missing things to PICKLE_SAFE. fixes #423

include graphite.intervals and collections.deque so that carbonlink
works
Fix 'now' handling in render queries

Previously, datetime.now() was called more than once
when rendering data.  This led to non-deterministic
selection of sample resolution.  For example, if
the schema is 60s:1d,300s:7d, and a request for
'-1d'->'now' came in, one would sometimes get
the 300s data and sometimes the 60s data, depending
on the length of time between the call to now() in
render.views and the call to now() in whisper.fetch.
Merge pull request #497 from justino/composer_from_url

Composer: create from URL
Merge pull request #531 from vimeo/fix-carbonlink

add missing things to PICKLE_SAFE. fixes #423
Merge pull request #523 from jcsp/consistent-now

Fix 'now' handling in render queries
Bring perSecond from master -> 0.9.12

The feature at:

   https://github.com/graphite-project/graphite-web/commit/3877366221ee93bfccabce736d69a4b11da25ea5

is too useful to leave in master, since 0.9.x is currently still in use.
Merge pull request #548 from graphite-project/bring-persecond-to-0.9.x

Bring perSecond from master -> 0.9.12