Fix Julia path and flags in Makefile

The Makefile of the JuliaProcessor plugin used
absolute file paths, which prevented compilation.
I have replaced the absolute path with a shell
macro as suggested by Julia's embedding
documentation.

Fixing this revealed a second problem: the linker
flags were incorrect. I have replaced the
offedning flag '-ljulia' with build flags
generated by julia, as per Julia's embedding docs.

See issue #80

Added Plugin Generator. Updated to JUCE4.