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
Allow format validators to be used on update