I downloaded the Webduino library, and was disappointed at what I found. A header file with no source file.
I copied your sketch and header and source files, and got the same errors you did.
I edited the WebServer.h file, and cut the implementation code, and saved the changes. I created a new file, WebServer.cpp, and pasted the implementation code there, added a #include statement, to include the header file, and saved.
Now, your sketch compiles perfectly. I've attached the modified files.
If I were you, I'd fire a nastygram off to the library developer and tell him what I thought of his or her being too lazy to create two files.
WebServer.cpp (20.1 KB)
WebServer.h (12.5 KB)