Cache policy attribute of uploader

Without such caching, calling #signature computes a new policy, which
may be different to a previous or subsequent retrieval of the policy, so
the signature may not match what is expected.
Add #clear_policy! method to clear cached policy string
Working on #113
Changing to use decode as final step.
Reverting a bit.
Wrapping branch.
Update for removal of let() helper
Removing extra \n
Bugfix: Use the 'mounted on' column for checking uniqueness.
Merge pull request #128 from avalancheau/cache-policy

Cache policy attribute of uploader
Merge pull request #132 from vinniefranco/multi-encode-issue-113

Multi encode issue 113
Merge pull request #136 from jazen/bugfix-use-mounted-on-column-for-checking-uniqueness

Bugfix: Use the 'mounted on' column for checking uniqueness.
Clean up README
Merge branch 'master' of github.com:dwilkie/carrierwave_direct
add changelog in preparation for version 0.0.14
backfill 0.0.13 and 0.0.12 changes to changelog
add all change links for each version
bump version to 0.0.14
update link in changelog