Make source and text files unexecutable
Add ErrorCode enum
Add undocumented (but well-known) error codes

Some error codes are not listed in:
https://dev.twitter.com/docs/error-codes-responses

But some error codes are well known to exist and exists on some
official documents. This commit adds such error codes to ErrorCode enum.

References:

*	https://dev.twitter.com/docs/auth/application-only-auth
*	https://dev.twitter.com/issues/1429
*	https://dev.twitter.com/discussions/27088
*	https://dev.twitter.com/discussions/14004
Merge pull request #70 from a4lg/fix/file-modes

Make source and text files unexecutable
Merge pull request #71 from a4lg/feature/error-codes

Add error codes