Consolidate bel subcommands into one

[abargnesi]

The bel sub-commands (e.g. bel translate, bel parse, etc.) are implemented directly in bin/bel. The bin/bel command calls out to other executables such as bin/bel2rdf.rb, bin/bel_compare.rb, etc.

We should:

• move Command classes to lib/bel/commands/ and refactor bin/bel
• move functionality in dependent bin/*.rb scripts to their respective Command class
• remove bin/*.rb scripts in favor of bel plus sub-commands