Began support for anonymous viewing of skill tree, and bug fixes

skill_tree.py
	-Can now load a skill tree guid from the URL
	-If no GUID is available, and the user is logged out, redirect him/her to the login page
	-Correctly sets HTTP headers to not cache the page on supported browsers

app.yaml
	-login is no longer required for the skill tree page.

css/main.css
	-added rules for a translucent and transparent overlay
	-cursor will no longer change on module divs

css/skilltree.css
	-z-index bug fixed for moduleInfo; it should no longer go under .moduleBoxes as easily.

datatypes.py
	-fixed typo in SkillTree Model
	-added a ComputedProperty semester_code in Module Model. This will now be used to sort for the latest info of a module.
		-using a double order will lag the datastore, therefore sorting with this ComputedProperty is more efficient

data.py
	-Data.get_latest_module() updated with semester_code
	-GetSkillTreeHandler returns more attributes for modules

home.py & templates/Home.html
	-homepage now properly detects that a student is logged in, and not merely an 'anonymous' logged in user.

scripts/jquery.ajax-progress.js
	-jQuery plugin to report progress of any retrieval/post via AJAX.

scripts/skillTree.js
	-more attributes are now loaded into each moduleBox DIV for display in the moduleInfo DIV
	-disableSelection jQuery plugin added, to disable text selection on elements. Currently implemented with modules in the module_list only.

scripts/script.js
	-added setModified() method, and isModified attribute to global variable, which will keep track of whether any modifications have been made since last save or load.
		-setModified() also alters UI as necessary
	-saveToServer() now accepts a callback, will be used for "Save & Share" function in the future.
	-loadFromServer() now causes the background to show a "loading bar" as it loads the skill tree from the server.
	-loadFromServer() now blocks UI interaction as the skill tree loads.
	-modules will now have an isLoading property added when they are being loaded from the server.
		-attempting to load a module while this is set to true will do nothing.
	-modules are no longer UPPERCASE in the skill tree.
	-#faculty_filter DIV is now emptied on page load
	-save button is now disabled on page load

templates/SkillTree.html
	-added "No Cache" headers in meta tags within the head tag
	-nested entire app in a background progress bar.
	-added #hiddenOverlay DIV to block UI interaction when necessary.