Returns the number of credits that you have remaining.
URL: http://www.bulksms.co.uk:7512/eapi/1.0/get_credits.mc
Example: http://www.bulksms.co.uk:7512/eapi/1.0/get_credits.mc?username=john&password=abcd1234
credit_count (floating point, to 2 decimal places, e.g. 2101.15)