Add folders to Nil/Ayin themes. Closes #17.

* Add sidebar_folders bool property -- enables sidebar folder icons in
place of the default arrows. This is undefined by default.
* Add colored_folders bool property -- if true, tints the sidebar icons
(both arrows and folders) purple to match folder colors. This is
undefined by default.
* Add folder.psd and folder@2x.psd to assets/src/.
* Add folder-*.png and folder-*@2x.png resources to assets/arrows/.

Also clean up some bits and pieces. Mainly comments.
Fix bug with tabs and window-shrinking.

If the window was resized in a certain way, the previous tabs could have
caused some sort of odd crash in ST2. So, the tabs are slightly modified
to prevent this. They're also probably visually a little different.

Also, fixed an odd 1px column missing from the active modified tabs in
Nil.
Fix tabset margins.
Renamed colored_folders to colored_folder_glyphs.

Figured this was a little less ambiguous since there's already
disable_colored_folder_labels, so colored_folders might be misread as
applying to that as well.
Update README. Replace many option sections with one.
Update screenshots.

About time, I know.