Mettle loader doesn't support local binaries

[smcintyre-r7]

metasploit-payloads has some extra logic in it's Ruby loading code that will allow Meterpreter binaries to be loaded from additional locations when Metasploit is installed. This is extremely helpful for developers that are debugging issues.

Right now Mettle supports loading from the data directory in Metasploit but not the user directory like metasploit-payloads does. We should

• Add the user directory for consistency
• Also update the Makefile to copy the binaries for testing like the metasploit-payloads one does