Re-design SDK structure, add support for v2018-08-01

[bandraszyk]

The sdk structure should be redesigned so it's more similar to nodejs sdk (https://github.com/voucherifyio/voucherify-nodejs-sdk/tree/master/src):

• common methods (utils) should be placed in separated file
• every namespace should be placed in separated file and it should contain methods only from that namespace
• add support for all methods from v2018-08-01 (https://docs.voucherify.io/v2018-08-01/reference) as breaking changes to validation rules has been introduced

Things to be done:

• change structure
• add support for v2018-08-01
• check python version compatibility, 2.x vs 3.x (add proper tests if needed)
• building and releasing automatisation: release new major version
• update readme