Alternate suggested default-command

This is the default-command I've been using for a while.  It works pretty well across platforms, which is nice.
Move the cross-platform configuration to Usage.md

This gives us more room to describe the caveats of this alternate
configuration.
Merge branch 'fixups/docwhat/patch-1'
Mention the MacPorts and Homebrew installation methods
Describe the "fine-grained" usage technique
Describe non-pasteboard uses

This is a in-repository replacement of the Other Uses page from the
GitHub wiki.
Describe if-shell usage that works in tmux 1.9
Fix comment typo
Prepare for a probable 10.10

We need room for the extra digit in the second part of the version.
Also add one for the third part too, just in case.