Make resh installation independent of location

[curusarn]

This project needs to be installed into ~/.resh/ because of the following:

• a line that gets added to .bashrc and .zshrc during installation
• absolute paths in the scripts
• absolute paths in the binaries

Proposal: use env variable to set the location.

[curusarn]

This is necessary for properly packaging this

[curusarn]

Improvements were made in #184
E.g.: #140

We now don't have everything in ~/.resh. Binaries could be moved elsewhere (anywhere in PATH). We only really need ~/.resh/shellrc.

[curusarn]

Sourcing of shell files relies on ~/.resh/ at the moment. We should install the shell files as a single file.