Install script doesn't check before adding install path to PATH env

[rffontenelle]

The installation script "install.sh" add tx's installation path to the PATH variable regardless if being in the PATH already. Running this script more than once (e.g. due to testing purpose or version update) causes multiple lines to be added into the detected profile (~/.bashrc, etc.).

I'd love to not have unnecessary entries added to my ~/.bashrc, or to be able to pass a flag disabling it somehow. :)

[keithbowes]

I figure the thought is that you'd only install it once and then run tx update to do version updates.