Add theme with unicode-7.0 symbols

- All powerline symbols, except for dividers, were replaced with Unicode ones,
  mostly from Unicode-7.0.
- Player statuses are using U+23Fx symbols. I have found it only in Symbola font
  though, so one may need to tell fontconfig to use it using something like

      <?xml version="1.0"?>
      <!DOCTYPE fontconfig SYSTEM "fonts.dtd">

      <fontconfig>
      	<alias>
      		<family>Terminus</family>
      		<prefer><family>Symbola</family></prefer>
      	</alias>
      </fontconfig>

- Modified indicator is using two characters: floppy disk and diacritics
  character that crosses it (WHITE HARD SHELL FLOPPY DISK + COMBINING REVERSE 
  SOLIDUS OVERLAY). It is not always displayed correctly.
- Vim cannot currently correctly handle Unicode-7.0 characters. [See this
  thread][1].
- `powerline.segments.vim.mode` got strings with identical width.

[1]: https://groups.google.com/forum/#!topic/vim_dev/zgY0r9UbKT4
Add troubleshooting question about powerline_unicode7 player symbols
Alias `blustery` to `windy` and not `foggy`
Make all modes have equal width in all themes
Mention powerline_unicode7 in documentation
Merge pull request #1222 from ZyX-I/update-themes

Add unicode-7.0 theme and some small theme fixes