Improve support for other shells like Fish

Fish Shell's $PATH variable contains directories separated by spaces
instead of using colons. Example:

bash -> /usr/local/bin:/usr/local/sbin:/usr/bin
fish -> /usr/local/bin /usr/local/sbin /usr/bin

To get around this inconsistency and others in different shells,
the $PATH is now extracted from the output of the 'env' utility,
which has the same consistent format independently of the
user's configured login shell.

fixes #6
Merge pull request #7 from aristidesfl/shells

Improve support for other shells like Fish