Move sp-pairs/tags to the front

These variables are used in somewhere above where they were
defined.
Declare optional dependencies
Fix implicit variable access in sp--get-substitute
Merge pull request #59 from tkf/compiler-warning

Fix compiler warnings
Handle both ? and \ character prefixes in lisp modes.
Fixed sp--skip-to-symbol-1 in modes where the pair delimiters weren't part of '(' syntax class (such as `' in latex)
[Issue #54] Fix the whitespace removal after sp-kill-sexp and sp-backward-kill-sexp