Added Blue variant

[BrianGilbert]

This is a fairly major restructure of the file structure but makes it a lot easier to add color variants without having to duplicate all the files, based of the Soda theme file structure

[joelfillmore]

I reorganized the files to better match the Soda structure. Following Soda, I went with "Flatland Dark" in case we ever decide to add a light theme as well.

I looked briefly through this pull request and it looks like the main change is the blue tabs. I wonder if it would be possible to add "flatland_blue_tabs" as an option, similar to how "flatland_square_tabs" was added? That would eliminate the overhead of trying to keep two sublime-theme files in sync.

[aapis]

What about a "flatland_tab_color" setting? It could (theoretically) support any colour as long as there are associated theme resources.

[BrianGilbert]

I have no problem with that however the tabs weren't the only images changed in my variant, I also changed the sidebar highlight colour.

Bear in mind that to do true colour theme changes you will probably need to change the icons for search as well as the file browser highlight.

[BrianGilbert]

see #62