Made some stylistic changes to the USM and bootstrap scripts.

 - I added a USAGE string to describe USM's options in short form, in
   addition to the long form available in `usm help`.
 - I corrected the use of `export USM_PATH=...`, which doesn't set
   variable in the current shell, but only in the environment. The
   correct form is actually `USM_PATH=...; export USM_PATH`.
 - Condensed expressions like:

    if FOO then;
        IS_BAR=1
    else
        IS_BAR=0
    fi

   down to:

    FOO
    IS_BAR=$?

 - Removed some excess variables
 - Removed pointless loop in `usm_ls`
 - Refactored the bootstrap script, and added more diagnostics.