Add spec templates
Add specs for Disk#read and Disk#index
Start specs for Disk#subsource
Add specs for Disk#subsource and Disk#close
Add specs for Rex::ImageSource::Memory#initialize
Add first specs for Rex::ImageSource::Memory#read
Finish examples for Memory#read
Add specs for Memory#close
Add specs for Memory#subsource
Add specs for Memory#index
Add specs for #read_asciiz and fix bugs there
Add shared examples
Fix grammar
Rescue an IOError on channel double-close.

This was causing output from python meterpreter
commands run on OSX to be discarded when the error
was raised, making cmd_exec not-so-useful.
Land #3846, Rex::ImageSource specs
Land #3933 - Fix cmd_exec with Python Meterpreter on OS X