Update signature verification page

The signature verification page at https://www.torproject.org/docs/verifying-signatures.html.en needs some work. A number of users have written the help desk to say they found the instructions confusing. The bash commands can be simplified, for example, by removing the full filepath in front of gpg.exe (since GnuPG's filepath gets added to your PATH variable anyway upon installation). The screenshots Sherief made for https://trac.torproject.org/projects/tor/ticket/9843 could also be added.