Support customized RRD URLs

Use case: I would like to bypass PHP for serving the RRD files and allow
the webserver (nginx) to compress it to reduce load on the Raspberry Pi.
I could go through all kinds of URL rewriting, but it is much easier to
set `$CONFIG['rrd_url'] = 'rrd/{file}';` instead and add a corresponding
location + alias directive to the nginx configuration.

This depends on https://github.com/manuelluis/jsrrdgraph/pull/17 to
avoid breaking on '='.
Add README file

Documents requirements, installation instructions (obsoleting
doc/INSTALL) and performance hints. Added a nginx server configuration
example that optimises for performance.
jsrrdgraph: Fix parsing of rrdfiles containing escapes

This fixes parsing of URLs such as 'rrd.php?path=some%3Ffile.rrd', and
if people really insist, paths such as 'C:\bar.rrd' (must be written as
'C\:\\bar.rrd', add additional escapes for Javascript strings if needed).
Merge remote-tracking branch 'lekensteyn/custom-urls'
type/base: fix undefined variable rrd_url when graph_type != canvas
Refer to jsrrdgraph using git submodules

The library code is identical, this is an ideal case for git submodules.
Update submodule for rrdfile fix in DEFs

Necessary for commit 754658743893cdea912f4536fb3ca30a13ca340f ("Support
customized RRD URLs").