Back-end Cleanup and Add New Posting back-end

-Renamed MarketPostModel to MarketPost
-Added new handlers for models: Textbook/MarketPost/Post/CourseInfo
-Handler methods currently used but not implemented (TODO: implement
them)
-Changed prototype of Price from double/float to int
-Add New Posting page - Implemented communication between the View (js)
and Controller (c#)
-Add New Posting page - Dropdown for indicating a Buy/Sell Post and Book
Condition input box added to View
-Implemented/modified constructors and data members for the following
models: CourseInfo, MarketPost, Post, Textbook
-Removed unnecessary/redundant files