Fix the way MD5 used in the pad's name is created.

In the previous implementation, the unique source of 'randomness' was
Date().getMilliseconds(), which returns a 0-999 int. In order to avoid
conflicts on pad names, MD5 is created using rawfilename and
Date.getTime().
Merge pull request #5 from otetard/bugfix/filename_to_md5

Fix the way MD5 used in the pad's name is created.