Fix temporary URL creator

Refactoring out the scattered instance variables around the `Real` class
meant a number of `nil` values were hidden away in the temporary URL
code.

If the temporary URL key is in place then it would create invalid URLs
with invalid signatures.
Clean up `create_temp_url` implementation and docs
Fix for tests under Ruby 1.8.7
Additional documentation line
Skip temporary URL test under Ruby 1.8.7

Original implementation in fog (of which this is based) included the
same `encode_www_form` problem.

Support for 1.8.7 is disappearing real soon now so not concerned with
the failures in code that has never worked.
Merge pull request #14 from brightbox/fix_temp_url_signing

Fix Storage#create_temp_url implementation