Convert site.conf to JSON during build

docs: add D-Link DIR-505 (A1) and Buffalo WZR-HP-G300NH (and remove WZR-HP-G300N, it is not actually supported)

This will allow us to use its content from other languages than Lua as
well.

docs: fix typo

The timeout was calculated incorrectly (in each iteration the span between
the start time and the current time was substracted from the timeout
again), and would often become negative, causing warnings in the kernel
log.

sse-multiplex likes to get the header as fast as possible.

The statistics data is now cached for 5s, no need to send an event every
second. The new interval is 3s, which is coprime with the cache timeout.

Allows simple handling of cached events.

docs: Improve documentation of fastd and autoupdater config in site.conf

Fixes #589

docs: releases/v2015.2: add ubiquiti airgateway

On the status page statistics section counters for transmitted and received traffic were mixed up. Transmitted traffic was shown as received while received traffic was shown as being transmitted.

Fixes #586

OpenSSL: Added source/old to PKG_SOURCE_URL

OpenSSL moves old versions of the library from http://www.openssl.org/source/
to http://www.openssl.org/source/old/1.0.2/ breaking the old links. That
behavior breaks the OpenWRT-build every time OpenSSL releases a new version.

This patch adds http://www.openssl.org/source/old/1.0.2/ to the PKG_SOURCE_URL
of OpenSSL to avoid breaking the build whenever OpenSSL releases a new
version.

Fixes #579

docs: add v2015.2 release notes