Change path of the page’s template

You can place the page templates wherever you want (also in a separate folder) as long as the files are within the current or parent theme’s directory.

UPDATE: Follow this link to get a more detailed information about the Template File Folders: http://codex.wordpress.org/Page_Templates#File_Folders

Take also a closer look at the Page Templates and the default TwentyTwelve Theme: their Custom Page Templates are stored within the page-templates folder.