Add Tizen Filesystem API.

This commit implements most of the Tizen Filesystem API.

Some things were intentionally left unimplemented, such as listing and
obtaining stoarage information. These will be implemented as soon as I get
access to a Tizen device and get the SDK up and running.

File I/O works, as evidenced by the new filesystem.html example (a crude
text editor) -- although this example doesn't showcase the whole API.