Add default client, accessible through TempoDB module

[gevans]

I've added the ability to use a default client in applications which have only one database and a single set of keys. In one particular use case, in a Rails initializer, one would be able to use:

TempoDB.configure do |config|
  config.database_id = ENV['TEMPODB_DATABASE_ID']
  config.api_key     = ENV['TEMPODB_API_KEY']
  config.api_secret  = ENV['TEMPODB_API_SECRET']
end

Standard TempoDB::Client instance methods can then be called as class methods of TempoDB:

TempoDB.write_multi(a_bunch_of_data_points)

Edit: The test suite appeared to be broken when I initially forked the repo. I took extra care not to make any changes to TempoDB::Client until I have time to see what's up with that. The changes I've made do have specs however. :)