Port to POSIX shell

So that it can be used on systems without bash / where bash is not used
for startup because of speed.
Merge pull request #22 from singpolyma/port-to-shell

Port to POSIX shell
Update tests to run the shoreman.sh script directly

Previously the tests were always running in bash, this change ensures
that the tests are run under whatever shell is specified in the shebang
line.
Set a default PORT

This is a very common parameter to want -- so maybe not crazy to have
a default?
Run commands as the user's default shell

Use the `$SHELL` variable to configure the shell that we use to run
commands. Also fixes a small issue where the `date` binary was not being
found properly, so this just hard-codes in the full path. Couldn't get
this working on OSX 10.9 or Ubuntu 12.04 without these changes.