PHP 5.3 Namespaces, PSR-4 Autoloading, Class-based Constants

Ok this is a brand new PR that is a rollup of all the changes requested from #247. This PR includes the following:

 - Proper PHP 5.3 namespaces using phpseclib as the root namespace
 - PSR-4 auto-loading support
 - Converted all global constants to class based constants
 - Updated all inline documentation to reflect namespaces

(I removed all of the visibility declarations from the original PR)

Feedback is welcomed and appreciated. Unit tests still need to be updated, and I have only tested the SFTP and SSH2 functions in very limited use cases, so there may be new bugs introduced that I have not seen. Please let me know if you find any and I can make any necessary tweaks.