Add support for querying multiple blasts

[stefan-pdx]

Currently, the gem only supports pulling blast info for a given blast_id. It would be nice to support a get_blasts method (or something similarly named), which returns a list of blasts given a date range per the API spec (start_date, end_date).

I currently get around this by using:

# Get a list of blasts sent out yesterday
options = {
  status:      :sent,
  start_date:  (Date.today-1).strftime("%D"),
  end_date:    (Date.today-1).strftime("%D")
}

client.api_get(:blast, options)

[jcrivera]

@slnovak - We'd like to create some kind of get_blasts method that will get blasts that meet criteria given in an options hash - maybe not specific to date ranges, but could be used for that. I'm concerned about a proliferation of get methods for every type of search option param. We will discuss a solution to this and would also welcome your help on a pull-request. We'll be in touch with details. Thanks.