[boost] status of type traits addition (has_operator_xxx)

Here is the status of the type-traits addition:
- I added the following operators: +=, *=, -=, /*, %=, &=, |=, ^=, <<,
>>, >>=, <<=, prefix ++, prefix --, postfix ++, postfix -- so that the
covered operators are now:
- I added in the doc. the fact that private operators are not covered
and lead to compile time errors
- for operators on built-in types that require integer arguments (%,
bitwise op.): I replaced a long list of specialiazation by an
automatic detection

Remaining work:
1. check what happens for pointer arguments,
2. implement trait for:
  . operator=
  . has_address_of_op<T>
  . has_dereference_op<T>
  . has_member_access_op<T>
  . has_subscript_op<T1 [,T2]>
  . has_pointer_to_member_op<T1 [,T2]>
  . has_comma_op<T1 [,T2]>
  . has_function_call_op<T,R [,P1, ...]>
3. have an MPL predicate for return value

As I have only little time (full time work and 3 young children...), I will:
- certainly do 1.,
- I will certainly leave 3. to somebody else in the future because I
am not enough agile with MPL,
- as for 2., 3 possibilities:
  (i) I do it but it will probably postpone the addition for a long time,
  (ii) I just leave it and I know is is a shame not to provide all
operator detection (nobody's perfect) or
  (iii) somebody else agrees to add them and have them reviewed at the
same time.

Frédéric