add wide string support using std::wstring class

following features are included:

  - transparent support of std::wstring for simple var in/out
  - transparent support of std::wstring for vector based in/out
  - transparent support of std::wstring for row (dt_wstring type)
  - new connection options for large content TEXT/NTEXT limits
  - superset capabilities for text longer than connection limits
  - avoid driver/database truncation of large text during write
  - large text handling affects std::string and std::wstring

simple commented example code:
------------------------------

// connect string
std::string dsn = "Driver={PostgreSQL ODBC Driver(UNICODE)};";
dsn += "Server=localhost;Port=5432;Database=odbc;Uid=test;Pwd=test;";

// connect params
soci::connection_parameters params("odbc", dsn);

// SQL_DRIVER_NOPROMPT
params.set_option(soci::odbc_option_driver_complete, "0");

// 200000 chars max TEXT/NTEXT size
// default: 32000 chars
params.set_option(soci::odbc_option_max_text_length, "200000");

// avoid truncation of long string storage into TEXT/NTEXT columns
// (optional, default is already 254 chars)
params.set_option(soci::odbc_option_fix_trunc_above, "254");

// create session
soci::session sql;
sql.open(params);

// query string and result definition
std::string query;
std::wstring content;

// fetch a 30000 char memo (NTEXT column)
query = "select memo_column from text_table where text_id = 1"
{
  soci::statement st = ( sql.prepare << query , soci::into(content) );
  st.execute(); //content successfully read cause less than 200000
}

// fetch a 2 million char memo (NTEXT column)
query = "select memo_column from text_table where text_id = 2"
{
  soci::statement st = ( sql.prepare << query , soci::into(content) );
  st.execute(); //-> exception cause max size is still 200000 !
}

// fetch a 2 million char memo (NTEXT column) again
// with special guided size (superset capability)
content.resize(2000000+1);
{
  soci::statement st = ( sql.prepare << query , soci::into(content) );
  st.execute(); //-> content successfully read, proper predefined
}