Added SCON configuration format parser

- This new format is an enhancement of boost's INFO file format.
- Moved variant_tree::read_info into variant_tree_parser.hpp
- The new variant_tree_parser.hpp now has support for
  INI, XML, SCON, INFO file formats
- Implemented basic_variant_tree<CharType>, which is now the base
  of variant_tree.

The SCON format is read/written by utxx::read_scon() and utxx::write_scon()
functions defined in variant_tree_parser.hpp.

Example of SCON configuration file format:
==========================================

test {
   key1 = "value1"           % This is a comment
   key2    value2            % Equal sign is optional as well as double quotes
                             %   (in the absence of double quotes the data type
                             %    is inferred)
   key3 = 3,  key4 = 2.0     % Key/value pairs can be separated by ','
   key4 = "non-unique"       % Keys don't have to be unique
   type address {            % Child nodes can have values (e.g. "address")
       encoded = true        % Supported value types: str | int | double | bool
   }

   key5 { k6="ttt", k7=100 } % In-line notation
   key8 { k9 "abc", k10 50 } % In-line notation without '=' sign

   #include "file.conf"      % Supports inclusion of other files at any level
}