Fix test warnings for deprecation (issue 3885)

https://code.google.com/p/otwarchive/issues/detail?id=3885

I got tired of seeing the giant deprecation warnings. This fixes:
- the vendor/plugins warnings (acts_as_commentable and livevalidation have been moved into lib/ and the vendor/plugins folder is now empty)
- the previously defined constant PDF warning (pdf mime type is now being defined by rails)
- one instance of the iconv warning, in the CSV download for challenge signups. 

The other uses of iconv are related to work downloads and are trickier to replace because they rely on iconv's transliteration features. Instead of investing a lot of time into fixing that, we just want to switch to using calibre for our download generation instead. 

- removed fakeweb which has been deprecated by vcr and replaced it with typhoeus

This also adds the gem 'pry' to our test and development environments where it is massively useful for debugging.