Explicit contructor + rvalue reference setter

Made the constructor explicit to avoid accidental implicit conversions.
Also added setting which accepts an rvalue reference to save a copy!
Fixed getStringIndexAt + cleanup

FixedImageFont::getStringIndexAt now reports the correct string index
for the given x value. X values too far to the left report index 0,
while X values too far to the right report the end of the string.
Merge pull request #4 from zackthehuman/master

Fixed for FixedImageFont + enhancements for FunctorActionListener