New class constants

WARNING_LEVEL = 'High',
MEASUREMENT_SYSTEM = 'English',
WEIGHT_UNIT_MAJOR = 'lb',
WEIGHT_UNIT_MINOR = 'oz',
CURRENCY_ID = 'USD'
Added LINEAR_UNIT as class constant
New class constant and methods

* Added DATE_FORMAT as constant
* Added new methods ScheduleTime & convert_date
Set default values for date related functions
Removed request body.

Set request using various class methods when using