Fixing in-place sed command on OSX (backup extension required).

OSX's sed flavor is slightly different from the standard unix. Usually,
if you omit a backup extension during in-place operations no backup
files will be created. On OSX you must instead specify an empty string.

More details at http://stackoverflow.com/questions/4247068/
Sed OSX/Linux compatability fix.

GNU sed requires -i'' (without a space).
BSD sed (OSX) accepts that if the -e flag is used.

Test Plan:
echo "abcd" > a.txt
echo "cdef" > b.txt
sed -i'' -e "s/cd/CD/g" "a.txt" "b.txt"
Switching from sed to perl for in-place replacing.

Turns out there's no way to reconcile GNU and BSD sed syntax for -i.
However, perl is available everywhere.

Test Plan:
echo "ab" > a.txt
echo "bc" > b.txt
perl -pli -e "s/b/B/g" "a.txt" "b.txt"
cat a.txt b.txt
Merge pull request #5 from alopatin/master

Minor fix to sed command on OSX