Added two new sorting functions and a way to increase/decrease priorities easily

See README for more details:

<leader>s@ - sort by @contexts
<leader>s+ - sort by +projects
<leader>j  - decrease priority
<leader>k  - increase priority
Removed some commented-out test code
Merge pull request #19 from ha17/dev-sorting-priority-changing

Dev sorting priority changing
Easy to add a priority to the current line

- several new leader keys to add priority to the current line
- updated README
Merge branch 'dev-add-priority' of https://github.com/ha17/todo.txt-vim into dev-add-priority