Add option for specifing a list of MFAs to skip when checking a module's specs

PropEr's tight integration with Erlang's type system is very helpful.
However, there are times when one would like to check all of a
module's exported function specs except for a handful of specs.  The
'{skip_mfas, [<MFA>]}' option can be used for such times.
Merge pull request #57 from norton/norton-skip-mfas

Add option for specifing a list of MFAs to skip when checking a module's specs.
Put files in alphabetical order
Fix misunderstanding in handling of binary types
Extend the text and the list of those mentioned
Two binary tests and two bitstring tests (the latter commented out for now)
Merge branch 'master' of git://github.com/manopapad/proper
Merge with upsteam.