Added some task descriptions so they show up with 'rake -T'

I guess it does no harm to have some additional task descriptions
and help our memory.

Before:

    rake changelog  # Update the changelog since the last release
    rake console    # Open an irb session preloaded with this library
    rake test       # Run the mocked tests
    rake yard       # Generate YARD Documentation

Now:

    rake build      # Build fog-1.8.0.gem
    rake changelog  # Update the changelog since the last release
    rake console    # Open an irb session preloaded with this library
    rake gemspec    # Updates the gemspec and runs 'validate'
    rake test       # Run the mocked tests
    rake validate   # Run before pushing out the code
    rake yard       # Generate YARD Documentation

Didn't want to describe all since some of them look potentially
unsafe for non core devs. I can document them otherwise.