Fix Solaris build

• fixed Solaris build, thanks to Walerian Sokołowski
• use poll() instead of select(), thanks to Walerian Sokołowski
• add the process_exists() API
• fix race conditions in a number of tests

Refreshing the code

• required in new R: explicitly register native symbols
• fix: make Ctrl+C work in Windows

MacOS support, API updates, bugfixes

• support for Mac OS
• shared read from both stdout and stderr of the child process
• new process_close_input() call to close the write end of child's standard input pipe; this in most cases will let the child know it should exit
• renamed process_poll() to process_wait()
• bugfix: group terminate in Windows

Initial release to CRAN

0.7.2

0.7.2 as uploaded to CRAN