Test new channel dump_image attribute in configuration files

https://bbpteam.epfl.ch/project/issues/browse/BVWS-94

Tests both as a global attribute and as a channel attribute.
Testing channel attribute dump_image actually dumps images to filedisk.
Taking review commens into account:

* Improve dump image tests by checking the size of the generated file
Merge pull request #266 from julitopower/master

Test new channel dump_image attribute in configuration files
Merge pull request #267 from julitopower/dump_image_test

Testing channel attribute dump_image actually dumps images to filedisk.
Merge branch 'master' of https://github.com/Eyescale/Equalizer
CR #267