Fixed bug in token parsing loop

[warezthebeef]

When parsing a template such as

{% if test %}{{ test | size }}{% else %}0{% endif %}

LiquidBlock.php:46 while ($token = array_shift($tokens)) loop was failing because $token = array_shift($tokens) evaluates as 0 on the second to last token and causes the while loop to end prematurely. Modified to evaluate the tokens array using count($tokens) and moved the $token variable assignment inside the while loop.

[warezthebeef]

I've updated this pull request to include some more changes, first the include functionality has been modified to match current Ruby liquid as outlined in file_system.rb. Basically this means the extension is .liquid (rather than .tpl) and all partial files must be prefixed with an underscore.

file_system = Liquid::LocalFileSystem.new("/some/path")

file_system.full_path("mypartial")       # => "/some/path/_mypartial.liquid"
file_system.full_path("dir/mypartial")   # => "/some/path/dir/_mypartial.liquid"

WARNING: While this now matches Ruby liquid it will break functionality for people who are already using .tpl partials

Next I've modified the if statement to support and/or logic as well as elsif blocks. So statements such as the following are now valid

{% if test1 and test2 %}
if works
{% elsif test1 or test2 %}
elsif works
{% else %}
else works
{% endif %}