dvtm 0.13

Updated dvtm to 0.13. Had to do a little inreplacing to work around OSX selectively removing SIGWINCH/VLNEXT in signal.h/termios.h when -D_POSIX_C_SOURCE is defined. Added a simple test.
Two character indentation here. Also, can this test be modified to do something more substantial than e.g. `--version` or `--help`? See `cmake.rb` for an example of an application formula with a good test and `tinyxml2.rb` for an example of a library formula with a good test. Thanks!