Add CL handling to hyc

This adds real command line handling to 'hyc' for issue #256

This fix catches missing/unreadable files and prints a nice
error message instead of a nasty stack trace when trying to
compile a non-existent file.

Also add this non-existent file check to hy to prevent the
current stack trace from something like "hy foobarbaz" when
"foobarbaz" doesn't exist.

also changes the failure return value to 2 to match Python.
documentation for defun / defn and let
documenting defn / defun
minor fix
Merge branch 'master' into pr/271
Add "spy mode" to REPL
With the command-line option --spy, the Hy REPL prints the Python
equivalent of each expression before evaluating it.
Merge pull request #265 from rwtolbert/hyc_args_fix

Add CL handling to hyc
Merge branch 'master' into spy-mode
Merge pull request #275 from khinsen/spy-mode

Add "spy mode" to REPL