first pass at cleaning up the tumblr migrator

1.  What is the "at" method?  I don't have it:
```
[mattrose@oscar public_html]$ ruby -rubygems -e 'require "jekyll/migrators/tumblr"; Jekyll::Tumblr.process("http://folkwolf.tumblr.com", true)'
Page: 1 - Posts: 50
/usr/lib/ruby/gems/1.8/gems/jekyll-0.11.2/lib/jekyll/migrators/tumblr.rb:72:in `post_to_hash': undefined method `at' for #<Hash:0x7f56c4d622f8> (NoMethodError)
	from /usr/lib/ruby/gems/1.8/gems/jekyll-0.11.2/lib/jekyll/migrators/tumblr.rb:27:in `process'
	from /usr/lib/ruby/gems/1.8/gems/jekyll-0.11.2/lib/jekyll/migrators/tumblr.rb:27:in `map'
	from /usr/lib/ruby/gems/1.8/gems/jekyll-0.11.2/lib/jekyll/migrators/tumblr.rb:27:in `process'
	from -e:1
```

2.  I think somewhere along they changed a conversation from a hash with "line" to a straight array
```
[mattrose@oscar public_html]$ ruby -rubygems -e 'require "jekyll/migrators/tumblr"; Jekyll::Tumblr.process("http://folkwolf.tumblr.com", true)'
Page: 1 - Posts: 50
Page: 2 - Posts: 50
Page: 3 - Posts: 50
/usr/lib/ruby/gems/1.8/gems/jekyll-0.11.2/lib/jekyll/migrators/tumblr.rb:86:in `[]': can't convert String into Integer (TypeError)
	from /usr/lib/ruby/gems/1.8/gems/jekyll-0.11.2/lib/jekyll/migrators/tumblr.rb:86:in `post_to_hash'
	from /usr/lib/ruby/gems/1.8/gems/jekyll-0.11.2/lib/jekyll/migrators/tumblr.rb:27:in `process'
	from /usr/lib/ruby/gems/1.8/gems/jekyll-0.11.2/lib/jekyll/migrators/tumblr.rb:27:in `map'
	from /usr/lib/ruby/gems/1.8/gems/jekyll-0.11.2/lib/jekyll/migrators/tumblr.rb:27:in `process'
	from -e:1
```
3.  Put an upper bound on the slug.  This prevents this error.
```
[mattrose@oscar public_html]$ ruby -rubygems -e 'require "jekyll/migrators/tumblr"; Jekyll::Tumblr.process("http://folkwolf.tumblr.com",format = "html",false,false,false)'
Page: 1 - Posts: 50
Page: 2 - Posts: 50
Page: 3 - Posts: 50
Page: 4 - Posts: 50
Page: 5 - Posts: 50
Page: 6 - Posts: 45
/usr/lib/ruby/gems/1.8/gems/jekyll-0.11.2/lib/jekyll/migrators/tumblr.rb:37:in `initialize': File name too long - _posts/tumblr/2012-01-19-five-cyclists-set-out-on-a-morning-ride-from-kanata-ontario-to-pakenham-and-back-three-miles-into-their-ride-luangpakham-drifted-into-the-bicycle-lane-he-hit-one-of-the-cyclists-and-then-another-and-another-and-another-and-another-he-hit-all-five-cyclists-and-continued-driving-luangpakhams-excuse-he-thought-he-had-hit-a-pole.html (Errno::ENAMETOOLONG)
	from /usr/lib/ruby/gems/1.8/gems/jekyll-0.11.2/lib/jekyll/migrators/tumblr.rb:37:in `open'
	from /usr/lib/ruby/gems/1.8/gems/jekyll-0.11.2/lib/jekyll/migrators/tumblr.rb:37:in `process'
	from /usr/lib/ruby/gems/1.8/gems/jekyll-0.11.2/lib/jekyll/migrators/tumblr.rb:32:in `each'
	from /usr/lib/ruby/gems/1.8/gems/jekyll-0.11.2/lib/jekyll/migrators/tumblr.rb:32:in `process'
	from -e:1
```