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
tile layer: added horizontal and vertical offset properties
map reader: read horizontal and vertical offset for tile layers
map writer: write horizontal and vertical offset for tile layers
property browser: added horizontal and vertical offsets for tile layers
isometric renderer: first take at rendering offseted layers
map renderer: added method to calculate the max offsets from all the layers
isometric renderer: use the max layers offsets to correctly calculate the map size
map: moved the logic to calculate offsets to the map instead of the renderer
layermodel: recalculate map offset on layer offset change
tilelayer: margins should be positive
changetilelayer: merged both offset commands into one to fix a bug when setting the offsets
maprenderer: fixed implementation of screenToTile and implemented tileToScreen using offset
isometricrenderer: correctly draw the offseted tile layers
maprenderer: screenToTile and tileToScreen accept Layer instead of TileLayer
maprenderer: the grid is drawn in an offset according to a given layer. Implemented only for isometric.
isometricrenderer: include drawing margins in the map size