Added modifiable output

Still 9 lines, however you are able to change now what characters are printed out. Just hand over two arguments:
./life "\\" "." for a backslash as "living cell" and a dot as dead cell. If no arguments are given, the original version, with "X" and " ", is used.
Deleted redundant, empty line.
Fixed compiling warnings.
Merge pull request #3 from dfmogk/master

Added modifyable output.