New script to calculate average blocktime between block heights

To use the script:
    ./contrib/avg_blocktime.pl DRAGONX

It has some default block heights defined, if you want custom ones
than those are given as 2nd and 3rd arguments. Currently the script
prints out:

Avg blocktime over 3000 blocks = 36.0526666666667

So with this evidence it seems that DRAGONX has the correct average
blocktime and network hashrate.

contrib/block_time.pl

@@ -5,7 +5,7 @@
 use warnings;
 use strict;
 
-# Given a block time, estimate when it will happen
+# Given a block height, estimate when it will happen
 my $block      = shift || die "Usage: $0 123";
 my $coin       = shift || '';
 my $hush       = "./src/hush-cli";
@@ -30,7 +30,7 @@ if ($block <= $blockcount) {
     if ($coin eq 'DRAGONX') {
         $minpb = 0.6; # minutes per block
     } elsif ($coin) {
-        # TODO: support custom bloctimes
+        # TODO: support custom blocktimes
         $minpb = 1; # assumes default blocktime of 60s
     }
 	my $minutes = $diff*$minpb;