changed default pair for "`" from "'" to "`"
removed special cases for "``" pairing as it is default now
added exceptions for lisp and TeX modes, those still need "`'" pairs.
Merge pull request #75 from vderyagin/backticks

Made backtick pair with itself, except in lisp and TeX modes
Update tests
Updated tests
Merge branch 'delete-selection' of github.com:Fuco1/smartparens into delete-selection