Gss api delegate (ParallelSSH#22)

Updated changelog.
Added function for setting GSS API delegation option on session. Added GSSAPI authentication tests.
Updated import privkey function signatures.
Added agent authentication test. Updated import privkey functions to default to empty passphrase.

Channel dealloc (ParallelSSH#19)

Fixed issue with channel double free when session open channel fails.
Updated exception handling for session authentication.
Updated tests.
Removed unused exceptions.
Added documentation.
Updated readme, changelog.

OSX and Windows wheels (ParallelSSH#18)

Added OSX wheel builds for 10.14 and 10.15 on Py3.8
Added Windows wheel builds for Python 3.6/3.7/3.8

Added sdist upload (ParallelSSH#13)

Updated CI cfg (ParallelSSH#14)

Added verion fix for manylinux (ParallelSSH#17)

Updated static files tracking (ParallelSSH#16)

Updated ci filters (ParallelSSH#15)

Updated CI cfg (ParallelSSH#14)

Added manylinux binary wheel builds on travis-ci.

Added manylinux dockerfile, dependencies and scripts.
Updated travis cfg, libssh build script.
Added changelog.