Changes to support building on Mac OS X and against Ruby 2.0

This is adding darwin12 to the list of architectures in exconf.rb, and converting from Config to RbConfig.
Ignore the now-default test-driver file from autoconf
Clear out any code for Ruby < 1.8.7

Leaving 1.8.7 support in for the time being, but anything older than
this should be upgraded.
Clean up arguments object if function wrapper goes out of scope

Fixes #41
Fixes #42
Clean up Allocation Strategies

Moved the one static function used into the only file that uses it and
got rid of the XMalloc strategy. Nothing in Rice uses custom allocation,
and I've never seen any use of the Xmalloc.

Closes #10, #11
Rename Default_Allocation_Strategy to Default_Free_Function

Makes the function usage and naming consistent with what it really is
and jives with its fellow Default_Mark_Function.
Test and fix converting from Ruby to a single char

It was testing a 0-length string for some reason, tested that case, the
good case, and the raw int conversion third case.

Fixes #34