Fix memory leaks in SeExpr (now valgrind clean except for dlopen/sym stuff)

Fucnctions instance is now no longer mallocd()
Store all strings made by the lexer in an array that have lifetime
  contingent on SeExprFunc. This avoids memory leaks on parse errors.
  It also allows us to not copy strings in SeExprAssignNode and
  SeExprVarNode
Also dlclose() at destruction of FuncTable time
Merge pull request #8 from aselle/master

Memory leak pass