Make thread safe

[sue445]

api_key and access_tokenare depends on class instance variable.

All API methods are not thread safe :tired_face:

• https://github.com/asonas/chatwork-ruby/blob/v0.7.0/lib/chatwork/entity_methods.rb#L5-L19
• https://github.com/asonas/chatwork-ruby/blob/v0.7.0/lib/chatwork.rb#L41-L43

So I want to fix this.

Features (draft)

client = ChatWork.create_client(access_token: user.access_token)
client.get_me

Note

All class methods (e.g. ChatWork::Me.get) continue to support