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