Rewrote to work with webapi-parser

Major changes (BREAKING):

• Rewrote to work with webapi-parser (#14);
• New dependency: ajv;
• New dev dependency: webapi-parser;
• Dropped dependency: raml-validate;
• Removed RAMLVersion option (now inferred automatically);

Minor changes:

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