Add test to ensure base URL ends with a '/'.

Add test to ensure base URL ends with a '/' and add it if missing. The
rest of the function assume the base URL ends in '/' when building URLS
for API calls.
Add HTTP error message in Exception. Fix #5.

When the API respond with an HTTP core that's not 200 or 404 then the
expcetion thrown includes the message that was in the body of the
response. Fix issue #5.
Merge branch 'master' of github.com:Traackr/traackr-api-php