(PUP-1195) Fix is_numeric/is_integer when checking non-string parameters

I expect a function called "is_numeric" or "is_integer" to check if a
variable is an integer or a number even if the variable passed by isn't
a string nor a number at all. Otherwise we should call them
is_string_a_number and is_string_an_integer and we have then to remove
the check for .is_a?(Number) and .is_a?(FixNum)

now checking also if it is a hex or octal number

improved/corrected checking for integer

* checking against Integer instead of Fixnum so that
  also Bignum is matching
* now .is_a? Integer is done first so this is quiet fast

Now many types of numerics are recognized.

1. Float/Integer values (signed or unsigned, with exponent or without)
2. octal and hex check
3. except hex numbers and the "0." in a float lower than 1 can be prefixed
   with a '0'.

whitespaces shouldn't be allowed as prefix/suffix

string representation of numbers should not contain any type of
whitespace.. the user is responsible to clean a string before checking
it..

fix documentation and added more checks

tried to be 99.9% backward compatible

* for now the decission is post poned if hex and octal numbers
  should be allowed or not (is_numeric)
* native Bignum is now also a valid integer class

fix problem with old 1.8 ruby and Hash.to_s/Array.to_s

In ruby < 1.9 array and hashes would be recognized as numeric
if they have a special format:

1.8:

  [1,2,3,4].to_s = "1234"
  {1=>2}.to_s    = "12"

1.9:

  [1,2,3,4].to_s = "[1, 2, 3, 4]"
  {1=>2}.to_s    = "{1=>2}"
(PUP-1195) Rephrase documentation for is_integer and is_numeric

The documentation contained references to future decisions about
functionality. 
Text rephrased for clarity.