Fix for class collision

Commit for my issue: https://github.com/Mashape/unirest-php/issues/11

Adds namespace Unirest to all files.  Maybe the documentation needs a little update, or maybe you want to change the Unirest.php name slightly so the call would be Unirest/Request::get() instead of Unirest/Unirest::get().

I would love to continue using current commits of Unirest, but until it uses namespaces I won't be able to use anything except my branch.  Sorry if I didn't do this whole issue/pull-request thing right.  I haven't done much public github contribution before.