Add more params to web/spider

[postmodern]

Add more params to the web/spider worker to control the spider.

• limit (Integer)
• max_depth (Integer)
• delay (Integer)
• open_timeout (Integer)
• read_timeout (Integer)
• ssl_timeout (Integer)
• continue_timeout (Integer)
• keep_alive_timeout (Integer)
• proxy (URI) (eventually we will want to define a global HTTP proxy option for all web/* workers).
• referer (String)
• user_agent (String) (eventually we will want to define a global User-Agent option for all web/* workers).
• robots (Boolean)

The web/spider worker should also have a agent_kwargs method which builds a Symbol Hash based on the params, which is then passed to Ronin::Web::Spider.site(base_uri,**agent_kwargs).