begin async api. render, getTemplate, and node-loaders refactored
update express render method to match async API
update web-loaders to match the new async API
hook up basic compiled async, start converting tests
update some tests for async api
more test progress; clean up a bit
Add IE8 support to nunjucks.

This is a rebase of the pull request at:

  https://github.com/jlongster/nunjucks/pull/75

In general, the fixes amount to dealing with IE8's lack of Array.indexOf(),
Object.create(), Date.now(), and string indexing via array notation.
Fix for loops getting falsey but not undefined values
Merge pull request #93 from toolness/ie8-fixes-rebased

Add IE8 support to nunjucks (rebased)
fix perf regressions, include new benchmark script
Merge pull request #104 from mattbasta/loops

Fix for loops getting falsey (but not undefined) values
add jinja test/benchmark
make "this" be the context object in filters (fixes #109)
Merge pull request #110 from jlongster/context-filters

Context filters
initial merge of async work
minor work
parse expressions when parsing function signatures (fixes #106)
Merge remote-tracking branch 'origin/master'
fix conflicts merging in work from master
begin AST transformation library for async-ness