Installer pre-checking

[joho1968]

I ran into some similar problems to those in #13 with a fresh installation archive. I think many of these could be avoided if some installer pre-checking was added. Specifically in this case, I think the installer pre-check should include:

• curl_exec() must be enabled
• allow_url_fopen() must be enabled

And in any case, this message: "Unable to determine dependencies for Winter CMS. Your composer.json file is invalid" is "kind of" misleading. And in the case of not allowing curl_exec(), the rather "odd" error message that Winter CMS cannot connect to the market place (sorry, can't remember the exact wording) isn't entirely obvious :-)

Also, it'd be nice if there was a "retry" option at this failure point, that would let me skip entering all the details again after I've corrected whatever the issue might have been.

[bennothommo]

Some good ideas here @joho1968 - we'll definitely look into them for future versions.