Rewrote to work with webapi-parser

Major changes (BREAKING):

• Rewrote to work with webapi-parser (#23);
• New dev dependency: webapi-parser;
• Dropped dependency: xtend
• First argument to a default exported function is now an array of webapi-parser.EndPoint objects;

Minor changes:

• Updated dependencies' versions;
• Switched to const/let instead of var;