return zero as timestamp when it does not present in riak cache
minimize replica count for faster tile removal in riak cache
handle exceptions when storing/removing tiles in riak
add versions option to wms service

allows to limit the WMS services to specific versions
only init libproj when requested
issue deprecation warning on python 2.5
document deprecation of python 2.5
document setuptools and update virtualenv section for windows
fixed rst markup
update CHANGES for 1.7
fix syntax error, remove unused import, upgrade dependencies for travis ci
fix logging output for tile_walker cleanups
fix coverages with self intersection
Merge pull request #128 from ramunasd/cache-riak

Riak adapter improvements
update wms.bbox_srs to support fixed BBOX for each SRS
clip wms queries to max extent of requested SRS
add tests for clipped wms queries
use assert_bbox_equals for bbox comparision
make tests more robust for different PIL versions
fix error in expection handling for mapnik source