Make backquote work with the latest mruby. Fixed #75

run mrbtest outside of source tree.

Because IO class, which is the superclass of File class, includes it.

Current implementation of Enumerable#hash calls #each to collect
hash values of enumerated objects (then calculates the hash value
of the Enumerable object).  But IO#each is a method to read lines.
It is not expected that IO#hash reads all lines from the IO object.
Or even worse, program waits for the IO object to be readable if
it is a socket or something cannot be read immediately.

Add pointer casting

Add File.readlink

Add File.chmod

Add explicit cast from void* to struct mrb_io*

For compatibility with C++.

Support Array argument

Apparently, in mruby-process gem, Process is now a Module, not a Class

Enable option :in, :out, :err to IO.popen

Should use global variable

Test io setup failure

Some systems do not allow UNIX sockets from arbitrary directories.
Instead of trying to `#define SOCKET_PATH` correctly I assume the /tmp/
directory is accessible and use it.

I am seeing this test fail in some environments but can't determine why.
The extra information will help me determine the reason bind(2) is
failing.

Add installation instructions