Check the entry points before lifting. Add a CFG function listing option. (#247)

* Check the entry points before lifting. Add a CFG function listing option.

 o Make sure that the specified entry points exists
   in the supplied CFG file before lifting the code.
 o Allow the user to list the functions that have
   been saved in the CFG file (--list-cfg-functions).

* Fixed several memory leaks (see details).

 o The deserialized (protobuf) data was not being
   released.
 o The LLVM context was not being released, leaking
   resources for which it had ownership.
 o The NativeModule object was not being released.

* Command line options fix (see details).

The OutputFilename command line parameter can't be
empty because it is initialized to stdout by default
(see llvm::cl::init("-"). The is-empty check has been
removed because initializing it to an empty string
breaks the tests because they expect to grab the output
by redirecting the stdout stream.

* Use the correct list when searching for entry points.