Add explicit requires and remove load path

Thanks for providing a nice gem!
I tried to use it, but ran into problems with the require:

```
`<module:Namecheap>': uninitialized constant Namecheap::Api (NameError)
```

I looked into the issue and found in the doc of the glob method (http://www.ruby-doc.org/core-2.1.0/Dir.html#method-c-glob):
"Note that case sensitivity depends on your system (so File::FNM_CASEFOLD is ignored), as does the order in which the results are returned."

The problem was that the `api.rb` was listed after the `transfers.rb`. I changed that to explicit `require` calls and changed the require in the spec helper too.