Merge pull request #32 from tomhughes/master

Allow render_old to take a unix timestamp
Don't install the unit test program into bin directory
Improve diff-importing and tile expiry script
Make munin graphs support Z19 and Z20
Keep track of how much time is spent with rendering tiles from each queue
Add a munin graph to display the time spent rendering in each queue

This is useful to determine how many resources are used to render missing tiles
and how many to keep things up-to-date
Fix #38 clock_gettime needs -lrt on some systems
Another attempt at including mapnik include directories from mapnik-config

use mapnik-config --includes rather than --cflags (as was attempted in 23be036).

This should fix #35
Clean up mod_tile directory by moving source files into a common src/ sub directory

also move includes into includes/
Remove outdated files that haven't been touched in 5 years.
Fix typo in install path of makefile
Add another priority level (RenderLow) to accommodate rerenders after style changes

In addition to Render and RenderPrio, add another priority level of RenderLow.

The idea is that if you rerender all tiles by touching planet-import-complete and
thereby likely overloading the server's rendering capacity you want to prioritize rerendering
of tiles that were explicitly marked dirty due to data changes over rerendering
due to planet-import-complete updates.

This is achieved by splitting rendering requests by if they are "old", or "very old".
As the tile expiry sets back the modification date many years, mod_tile splits
rendering requests based on how old the tiles are. If they are no more than a
certain threshold (currently set to one year) old, they are rendered with lower priority.
significantly increase the load thresholds after which mod_tile reduces rendering

sets the new MaxLoadMissing to 50 and MaxLoadOld to 16.

With the prevalence of multi core servers today, the previous
default thresholds of 10 and 5 were becoming to lower than the average
CPU count. Furthermore, load is not an ideal measure, as it includes I/O
load and much of the I/O load might well come from serving rather than
rendering. Especially if the database is on a separate SSD. Lastly, using
the thresholds in mod_tile isn't super helpful, as due to hysteresis of
 rendering queues stop submitting tiles at a time of high load is not
particularly helpful without stopping rendering from the queue.
Update the readme.txt somewhat by deleting old and outdated content

This likely needs more improvements to make the readme actually useful.
But for now this mostly removes parts of the readme that were totally
out of date and confusing.