Oracle timestamp workaround added

• Added workaround for Oracle's timestamp(n) data type (#17).
• Support for writing long character columns improved.
  If column length is bigger then 65535, the data buffer is initially set to 65535 bytes and extended only when bigger data value is encountered.
• Oracle Express database added to tests.

Query timeout, support for dates and fixes in long string handling added

• R dates (Date, POSIXlt and POSIXct classes) are now handled automatically. It is no longer needed to manually cast them to character vectors.
• It is now possible to set query execution timeout.
  • sqlPrepare() and sqlExecute() function support additional query_timeout parameter
  • odbcSetQueryTimeout() and odbcGetQueryTimeout() functions were added
• Support for reading long character columns improved.
  • If the driver does not report column length, 65535 characters is read.
  • If the driver reports extremaly large column length (more then 16M), 16M characters is read.

varchar(max) column type support

Added support for column types reporting 0 as its length (most notably Ms SQL Server's "varchar(max)" column type).

• When passing data from R into a database, data of any size are supported (the available memory for ODBC buffers is the only limit).
• When reading data from database into R a default RODBC's buffer length of 255 characters is used to assure compatibility with RODBC (RODBCext relies on RODBC functions on data retrieval).
  This means strings of type "varchar(max)" will be truncated to 255 charactes.