Merge pull request #3 from dara123/master

add plan
updated readme with installation, usage, requirements, documentation and tests examples
Merge pull request #4 from adamaltman/master

updated readme with installation, usage, requirements, documentation and...
adjust metered billing and three d secure endpoints and added test
use constant for metered billing type, and fix classname of test
Merge pull request #5 from adamaltman/master

adjust metered billing and three d secure endpoints and added test
Update RebillySubscription.php
Merge pull request #1 from Rebilly/master

Merge PR