Add :base_url to HTTPClient configuration

api = HTTPClient.new(:base_url => 'https://api.example.com/v1')
api.get("/users.json") # => Get https://api.example.com/v1/users.json
api.get("https://localhost/path") # => https://localhost/path

Passing path to get, post, etc. is recognized as a request to
:base_url + uri.  If you pass full URL :base_url is ignored.
Add base_url property to HTTPClient
Merge pull request #218 from nahi/topic_base_url

Add base_url