Fix required for 458, paths that begin with './'

[jpfiset]

avoid name collisions with registered helper functions.

Issue #458

[kpdecker]

This seems like a useful change but I don't think it quite works as expected as this test is failing.

test('GH-458: Scoped this identifier', function() {
  shouldCompileTo('./foo', {foo: 'bar'}, 'bar');
});

[kpdecker]

And immediately after I make that post I see my mistake in the test :/ After fixing my error there everything is green.

Bonus here as this provides optimized output for the precompiler when not using known helpers mode.

require('./lib/handlebars').precompile('{{./foo}}')
'function (Handlebars,depth0,helpers,partials,data) {\n this.compilerInfo = [2,'>= 1.0.0-rc.3'];\nhelpers = helpers || Handlebars.helpers; data = data || {};\n var stack1, functionType="function", escapeExpression=this.escapeExpression;\n\n\n return escapeExpression(((stack1 = depth0.foo),typeof stack1 === functionType ? stack1.apply(depth0) : stack1));\n }'

require('./lib/handlebars').precompile('{{foo}}')
'function (Handlebars,depth0,helpers,partials,data) {\n this.compilerInfo = [2,'>= 1.0.0-rc.3'];\nhelpers = helpers || Handlebars.helpers; data = data || {};\n var stack1, functionType="function", escapeExpression=this.escapeExpression;\n\n\n if (stack1 = helpers.foo) { stack1 = stack1.call(depth0, {hash:{},data:data}); }\n else { stack1 = depth0.foo; stack1 = typeof stack1 === functionType ? stack1.apply(depth0) : stack1; }\n return escapeExpression(stack1);\n }'

Just merged this. Thanks!