Merged back from personal branch to trunk (svn merge -r846:940 ...)

Changes:
* Major enhancement to the Microsoft SQL Server stored procedure
heap-based buffer overflow exploit (--os-bof) to automatically bypass
DEP memory protection.
* Added support for MySQL and PostgreSQL to execute Metasploit shellcode
via UDF 'sys_bineval' (in-memory, anti-forensics technique) as an
option instead of uploading the standalone payload stager executable.
* Added options for MySQL, PostgreSQL and Microsoft SQL Server to
read/add/delete Windows registry keys.
* Added options for MySQL and PostgreSQL to inject custom user-defined
functions.
* Added support for --first and --last so the user now has even more
granularity in what to enumerate in the query output.
* Minor enhancement to save the session by default in
'output/hostname/session' file if -s option is not specified.
* Minor improvement to automatically remove sqlmap created temporary
files from the DBMS underlying file system.
* Minor bugs fixed.
* Major code refactoring.