Fix compilation of ODBC-specific SOCI header with new include paths.

If only soci/include directory is in the compiler include path, including
<soci-backend.h> doesn't compile, as it should be included as
<soci/soci-backend.h> or, as this patch does, "../soci-backend.h", which is
uglier but simpler as it doesn't require changing any SOCI makefiles.
Add helper cstring_to_double() and use it in PostgreSQL backend.

Replace PostgreSQL-specific and incorrect string_to_double() function with a
common (i.e. reusable) cstring_to_double() which always interprets the string
as being in "C" locale as this is what the actual strings, retrieved from the
database, use.
Merge pull request #237 from vadz/odbc-header-fix

Fix compilation of ODBC-specific SOCI header with new include paths.
Merge pull request #238 from vadz/cstrtod

Add helper cstring_to_double() and use it in PostgreSQL backend.