Add in PREFIX/etc/julia/juliarc.jl file for site-specific commands to be run at startup

Lays the groundwork for addressing #4222
Install platform-dependent juliarc files during `make dist` step
Add in juliarc.jl for OSX, remove environment setting in the Applescript launcher
Merge pull request #4351 from JuliaLang/sf/juliarc

Add in PREFIX/etc/julia/juliarc.jl file for site-specific commands