#47 Automagic path detection on Unix slow

[Thomas Holland]

Problem:
The automatic path detection done one the first AVR Plugin startup can take a very long time on Linux.

Solution:
Change the path detection to only use the 'find' command after asking the user to do so (and warning him that it may take a long time)

Also the path to the include files should be taken directly from 'avr-gcc' and not via an independent 'find'.