Compatibility fixes

- PHP4 doesn't allow method chaining
- $_SESSION isn't always defined
- on PHP5.1 and earlier using 0x100000000 instead of 4294967296 gives E_NOTICE
- array('Net_SSH2', 'func_name') doesn't work so we fix this by passing $this to it.
  to make that work on PHP4 pass by call-time reference. normally this would result
  in a fatal error on PHP5.4+ but doesn't seem to in this case. the following URL
  elaborates:

  http://stackoverflow.com/q/20732563/569976
SSH2: typos in last commit
SFTP: add explanation for use of 4294967296
Merge pull request #221 from terrafrost/php-compat-fixes

Compatibility fixes

* terrafrost/php-compat-fixes:
  SFTP: add explanation for use of 4294967296
  SSH2: typos in last commit
  Compatibility fixes