Option for JumpTrigger only during snippet expansion

Solves [Bug #1231677](https://bugs.launchpad.net/ultisnips/+bug/1231677).

I think two simple test cases would be enough for this feature:

1) Start a Vim instance without parameters, check that g:UltiSnipsJumpForwardTrigger and g:UltiSnipsJumpBackwardTrigger are mapped before and after an snippet expansion


2) Start a Vim instance setting g:UltiSnipsClearJumpTrigger, check that g:UltiSnipsJumpForwardTrigger and g:UltiSnipsJumpBackwardTrigger aren't mapped before and after an snippet expansion, and using i_CTRL-R_= check the mappings during snippet expansion.

I'm not sure how this could be implemented on test.py, and as I'm currently in ms-windows I don't have screen.
I can implement those test using [runVimTests](http://www.vim.org/scripts/script.php?script_id=2565) if you like.