Making default PHP namespace generic and fix issues 628 and 635

[who]

This fixes issue #628, which causes name collision.

[wing328]

@who, thanks for submitting a PR to make the namespace name more generic.

I was the one who temporarily hard-coded the namespace/package name. I was thinking about using the host name in the base URL to come up with a better namespace name. For example, if the base URL is http://api.foursquare.com, the package will be "foursquare-php".

Do you think this would be a better approach to name the namespace by default?

[who]

@wing328 Not everyone will use that naming convention, so might as well go with a generic one (hence "SwaggerClient") until it's fully customizable with a command line parameter, json setting payload, or by other means.

[fehguy]

Please chime in with any suggestions to #616

[dariusz]

+1 on just making it generic. ApiClient, SwaggerClient, etc. would be great. Anything other than PetStore :)