[e-acsl] generate nice variable names whenever possible

[e-acsl] bug fixed in functions with both a postcondition and multiple returns

* fixes bug:
  - fixed bug in quantifications when the bound variable got C type
  - fixed bug with DEV_FLAGS in Makefile.in
* implements new E-ACSL features:
  - invariant as assertion
  - existential quantification \exists over integers
  - equivalence <==>
  - \at as a predicate
  - conditional _ ? _ : _ (for terms and predicates)
  - binary boolean operations over terms
  - mixed assumes and ensures in function contracts
* other new features:
  - better error messages when runtime checks fail 
    (replace e_acsl_fail by e_acsl_assert)
  - remove option -e-acsl-assert
  - add E-ACSL manuals in doc/manuals
  - add files doc/Changelog, INSTALL, README and VERSION
  - new option -e-acsl-version
  - check Frama-C version at configure
  - make src-distrib
  - header 2012

[e-acsl] universal quantifications done (including testing)

[e-acsl] do not stop anymore when detecting an invalid/unsupported annotation
[e-acsl] tests for invalid quantifications

[e-acsl] universal quantifiers over integers. Not yet finished: work only in some cases
[e-acsl] logic variables (required by univ quantif)

[e-acsl] update todo-list

[e-acsl] tests now independent of GMP version (but GMP still required)

[e-acsl] fixing bug with logic label Here and Post

[e-acsl] all tests are now executed with gcc
[e-acsl] fix bug with integer division and modulo
[e-acsl] untested support of most logic labels
[e-acsl]

[e-acsl] support of \at for stmt labels

[e-acsl] support of field accesses in logic
[e-acsl] support of array accesses (yet missing guards) in logic
[e-acsl] support of pointer dereferences (yet missing guards) in logic
[e-acsl] support of \null in logic

[e-acsl] support of assumes without ensures in stmt contracts
[e-acsl] partial support of function contracts (the same subset than stmt contracts)
[e-acsl] additional test for nested code annotation
[e-acsl] code refactoring: visit.ml splitten into several compilation units

first step for support of statement/function contracts (statement ensures could work but yet untested)