Add some support for linux

Most of the effort has gone into differences with paths.

What works:

        export R_HOME=/lib/R
        fsharpi

TODO:
        Use `which` to find R when R_HOME not set
        There appear to be some problems when using fsharpc
Update RInit.fs

Accidentally left in some debug code
Merge pull request #57 from jpalmer/master

Code to make the type provider work in Linux