Use relative symlinks for single color tiles

Using a relative link allows the cache to be portable even when the
option to link single color tiles is used.
Use relative symlink when os.path.relpath is available

Python 2.5 does not support os.path.relpath so use an absolute path for
2.5 and relative >= 2.6.
print available coverages when referencing missing coverage in seed conf
add minimal and full seeding configuration; extend full_example
catch IOErrors when PIL/Pillow can't identify image file
Merge pull request #107 from walkermatt/symlink

Use relative symlinks for single color tiles
update CHANGES
Merge branch 'master' of github.com:mapproxy/mapproxy
fixed test for IOErrors from PIL/Pillow
update AUTHORS file
add release date of 1.6.0rc1 to CHANGES.txt
Allow debugging multiapp
Merge pull request #109 from vrsource/debug_multiapp

Allow debugging multiapp
fix ssl verification behind http proxy
try to fix python 2.6 compat for http proxy fix
handle connection errors in renderd client
upgrade to riak python client 2.x
ignore errors while symlinking single color tiles
upgrade riak version for travis
remove riak prefix, allow alter port numbers, update documentation