Embed the R directory in the compiled extension

If the user already informed the R directory while compiling the
extention, then it is not needed to explictly set R_HOME before using
the library.
Cleanup config.h during `make clean`
Changed Rakefile for new version of hoe
fixed problem where parts of r shutdown were called twice
exposed to R function
protected R objects from being freed by garbage collector when there is still a ruby reference
add hooks on garbage collection
Modify extconf.rb to look for R lib and headers in common directories as well as config (similar to mysql2 gem)
Use File.join to construct final path
Merge pull request #7 from Mechaferret/build_without_flags

Modify extconf.rb to look for R lib and headers in common directories as...
Merge pull request #8 from aaronjg/c1247f4148ab3540b0a17cc583f8c62790f23039

Added protection for R garbage collection when objects can still be accessed from Ruby
Merge pull request #6 from terceiro/r-home

Embed the R directory in the compiled extension
Merge pull request #10 from aaronjg/1cf57d989190b325552b7e24c21edf034eada1e6

Add Hooks for Garbage Collection