Bugfixes to #39 and fixes #40. Exported macros can be imported by

{% import "my-macros.tpl" my_macro, my_other_macro as renamed_macro %}.

A macro can be exported by adding "export" to the declaration:

{% macro my_macro() export %} ... {% endmacro %}

Exported macros can't be overriden by a user's Context.
Introduced new API-function: AsSafeValue.
Introduced new keyword: "export"