Add price field for each mixture. Closes #39.

* Add currency application.
* Add USD price field in mixture model.
* Add templatetag to convert currency value in template rendering.
* Add test for templatetag.
* Add default currency code to convert prices.
Merge branch 'feature-39-Add_price_value_for_products' into release-6
Merge branch 'develop' into release-6. Refs #39.

Conflicts:
	poppurri/mixture/models.py
Merge branch 'release-6' into develop