Added a USE_FHS_PLUGIN_PATH switch

Setting this variable during the qmake run will make tiled
correctly picking up the install dir of plugins if the
lib path differs from "../lib" (generally the case for 64 bit releases).
Merge pull request #621 from Ablu/64bit_plugins

Added a USE_FHS_PLUGIN_PATH switch
Make travis run the tests
Merge pull request #622 from Ablu/travis_tests

Make travis run the tests
Removed unused include

Found by Ablu. ;)