Add ability to override base url

[Andrey Sokolov]

Currently base URL is hardcoded. And for paid clients OWM advice to use another URL - https://pro.openweathermap.org . So there is lack of functionality to override base API URL.

Patch implementing it is in attach.

[Ashutosh Kumar Singh]

2.5.2.3.005: Fixed Issue #10 (Add ability to override base url)

→ <<cset 4df7497e3058>>

[Ashutosh Kumar Singh]

Hi, it is now fixed in OWM JAPIs v2.5.2.3. Thank you for your patch.

I have created a new class OWMPro for accessing the OWM's Pro APIs. I chose this design because there are few APIs available only in pro plans, and this class will be a place to host the code for those APIs along with the free APIs, of course. Thank you.