Satisify both $PATH and default pkg binary loc.

Per PR #23 , 'pkg' can be installed to another location, hopefully still in $PATH. Added a which() function to handle detecting this.
Per PR #27 , the default for FreeBSD 10.0-RELEASE and $PATH for 'pkg' will result in '/usr/sbin/pkg' if we use which. Given this, and it being a wrapper to '/usr/local/sbin/pkg', we have a stated default pointing to '/usr/local/sbin/pkg' if the file exists and is executable. Else default to the which() function and attempt path detection of the 'pkg' binary.
Trying to make peace with both #23 and #27. Will still need to look into parameritized passing of the pkg path for truly custom placement of binaries outside of $PATH (user customs)