Add cache object manager for RateSource, convert only if needed

[iXioN]

• The cache object manager allow to use memcached when you need to get a rate and avoiding hitting the db every times. Increase the perf a lot for pages with many prices conversion
• Convert the currencies only of the origin currency and the destination currency are differents
• Add a template tag that tallow to convert currencies directly on html files

[coveralls]

Coverage decreased (-19.4%) to 67.626% when pulling 6d97896 on iXioN:master into 966d116 on evonove:master.

[coveralls]

Coverage decreased (-18.0%) to 69.065% when pulling cfcfda5 on iXioN:master into 966d116 on evonove:master.