Multiple tweaks to vim configuration

- remap Y to yank until EOL rather than yank entire line
- add DiffOrig command to diff buffer content with original file
- remap CTRL-L to clear screen and search highlighting before redrawing
- remove <leader><leader> map to force screen redraw (use CTRL-L)
- subtly highlight long lines (>80) using solarized background tones
- jump to the last known cursor position when editing a file
- slightly neater vim status line (no square brackets around file type)
- remove CTRL-{h,j,k,l} mappings for window navigation (I never use them)
- general tidy up of vimrc and gvimrc (comments, indentation etc.)

Note: many of the changes are the result of me finally having a look
at the example vimrc files included with vim and shown on the vim wiki