Restrict `blocktrans` content to only strings and plain variables. #88

This matches Django behaviour.
Runtime blocktrans compiler and runtime.

Will generate runtime translation code only when 'blocktrans_fun'
isn't available on compile-time.
README updated. Some unit-tests added #88.
Fix (probably) README indentation.
Extract phrases from blocktrans. #88
Unittests to restrict complex content in blocktrans. #88
Merge remote-tracking branch 'seriyps/runtime-blocktrans' (#130)

Close #88.