Specify which attributes should be serialized
Reorganise config rspec
Filter the attributes which are written to the config file
Make config a member variable

Config is initialised only by reading the file, so re-creating it only
for the purposes of writing the file is pointless.

This will require modifying Config post-read with additional opts sent
to Client.
Add method and test for writing config from Client
Method and test for writing config file from YaST
Merge branch 'master' into review_140602_config_write_interface
Remove copy and paste error from apidocs
Allow options to override config file

Options passed to Client.new should persist when calling write_config
If options are set, they are stored in the Config after it has read any
config file.  This is necessary for YaST to pass in its registration
settings and persist them to /etc/SUSEConnect.
Make Travis happier
Readability and travis fixes
100% test coverage
Make initialize small again for Travis

Had to farm out some initializations to extra methods
More travis back and forth
Add check that non-serialized attributes are not.
Merge pull request #99 from SUSE/review_140602_config_write_interface

[Review] Request from 'wstephenson' @ 'SUSE/connect/review_140602_config_write_interface'