Eliminate assumption of ~/.mason as default location

[jfirebaugh]

We should eliminate ~/.mason as a default location and document the use of a git submodule as the recommended approach.

Related to #140.

[springmeyer]

:+1: mason should be able to be installed anywhere, and we should probably stick with unix conventions in the future: bin/mason for the command and share/mason/ perhaps for scripts? Then people could just put it on PATH or install locally (like with a submodule as you describe). All of mason's logic would be smart and relative.

[jfirebaugh]

eliminate ~/.mason as a default location

Done in #151.

document the use of a git submodule as the recommended approach

Still needs to be done.