Copy exceptions

[pkittenis]

• Adds re-raising exceptions on SFTP errors at SSHClient.copy_file and SSHClient.mkdir
• Updates gevent requirements for py2.6

[coveralls]

Coverage decreased (-87.1%) to 0.0% when pulling e0a0ba6 on copy_exceptions into 233a55e on master.

[coveralls]

Coverage decreased (-1.5%) to 85.6% when pulling e0a0ba6 on copy_exceptions into 233a55e on master.

[coveralls]

Coverage increased (+0.4%) to 87.467% when pulling e7193d7 on copy_exceptions into 233a55e on master.