property access speed improvement

[sreekotay]

• removed recursion
• removed array shift
• addeed property caching

[aishikaty]

@sreekotay Thank you for the pull request. Even if this speed optimization is perfectly valid I am not going to merge it into the master, as the main goal of the project is to keep the code as small as possible.

[aaferrari]

I discovered the following bug when testing the code of @sreekotay that does not occur in the original: When empty strings are passed the variables are replaced with null. For example, mustache("Hello {{name}}!", {name: ""}) returns Hello null! instead Hello !.