Add {out_dir, false} option to explicitly disable generating a .beam.

Fixes #128.
Fix error reporting.

Fixes #129.
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