Add wrapper for icr binary.

This will ensure that the $CRYSTAL_PATH environment variable gets
set properly, so Crystal will know where to find its libraries.

We also compile icr if it has not been compile yet.
Merge pull request #85 from booch/icr-wrapper

Add wrapper for icr binary.
Add a File::Stat class
Let Dir#exists use File#stat
Correctly parse "x = y rescue z" as "x = (y rescue z)"
Merge pull request #88 from prep/stat

Add a File::Stat class
Check return value of stat and lstat and raise Errno in that case. Added File::Stat.new(filename : String).
Removed colon after exception message.
Switch to MC-JIT. Now the codegen specs can use exceptions and trampolines.
Pass "execstack" flag to linker in Linux