handle threshold_res higher than first resolution
restrict tox dependencies
make fixture_dir configurable in SystemTests
refactored assert_img_colors_eq function
refactor imports

- remove duplicate __future__.print_function imports from python_modernizer
- remove __future__.with_statement imports (needed for 2.5 only)
remove 2.5 specific exception handling
add tests for MockServ
do not print assertion errors in MockServ
remove obsolte test code
init logging in autoconf
convert SRS codes to uppercase in capabilities parser
travis no longer supports python 2.5
catch and log sqlite3.OperationalError when storing tiles

this should suppress 'database is locked' errors that can happen under
high load. tiles are not stored in this case.
init logging in autoconf
(cherry picked from commit 6fac998c9cef8f25ab80c97546bac1b79400e8b8)
convert SRS codes to uppercase in capabilities parser
(cherry picked from commit dc0e57b47b003854c3e9e0f180e3a80e8eecb23c)
catch and log sqlite3.OperationalError when storing tiles

this should suppress 'database is locked' errors that can happen under
high load. tiles are not stored in this case.
(cherry picked from commit c484514793a281ced7eabb5d1ebf77468ed4e019)
fix 7cadd96 for python 3
add more tests for mock_httpd
fix check for jpeg magic bytes
do not open cascaded responses when image format matches