Refactor public API

[antonkomarev]

This PR contains:

• renaming create static methods to make because all it does - instantiates (making) new instance, and not persisting (creating) anything;
• renaming RocketChat::channel() to RocketChat::getDefaultChannel() to make its meaning more explicit;
• renaming RocketChatAttachment::setFromArray to RocketChatAttachment::setPropertiesFromArray to make its meaning more explicit;
• adds get prefix to all accessor methods;
• drops RocketChat::url() accessor since it's not meant to be used outside of the class.