Fixes #48. Added some AST-building improvements for faster execution
Improvements to the EXECUTION speed. API-Change: INodeEvaluator
removed and replaced by IEvaluator.
Fixes #39
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"
Respect AsSafeValue() in variable resolver for any *Value.
It's not allowed to export two macros with the same name.
It's not necessary anymore to apply the 'safe' filter on HTML filters.
Introducing template sets. Introducing Globals in template sets.
Globals is a context which is shared among all templates in a set.
You can override keys in Globals by using the per-template Context.
Fixes #35; API-Change: SetDebug() removed; ExecutionContext.Logf
introduced. Default template set is created on pongo2 init.
Fixed bug in ssi-tag.
Adding a base directory option to template sets. Fixes #41
Introducing template sandboxing (directories, banned filters/tags).
Fixes #49
Slight godoc fix.
Doc+README updates
Updated the benchmarks.
README fix
Added a test for macros calling macros.
New Error return type. API-incompatible change. Fixes #47
Failing test for #50.
Merge pull request #51 from ahall/master

Failing test for #50.
Still a failed test, but corrected the expected output.