Updated appveyor cfg. Added openssl and zlib static archives for wind… …ows builds.

Added libssh2 modules to package data. ResolvesParallelSSH#51. Added python 3.7 target to travis cfg. Removed OSX 6.4 builds from travis cfg - deprecated by travis-ci. Updated changelog.

Added libssh2 modules to package data. ResolvesParallelSSH#51. Added python 3.7 target to travis cfg. Removed OSX 6.4 builds from travis cfg - deprecated by travis-ci. Updated changelog.

Handle EAGAIN for channel and sftp writes correctly and return rc, by… …tes_written tuple so clients can resume from last written byte on next call to write. Updated docstrings for channel write functions, tests, travis cfg, flake8 configuration for sftp.

Updated readme, installation instructions in documentation. Updated setup.py for readthedocs builds. Updated travis cfg. Updated PyPi classifiers.

Merged and updated OSX travis configuration. 

Updated travis cfg 

Updated travis cfg 

Updated travis cfg 

Updated travis cfg