Reimplementation (#1)

* refactor into separate modules

* update README

gpg/exceptions.py

@@ -0,0 +1,37 @@
+"""Exceptions which may be raised during execution of gpg wrapper functions."""
+
+
+class GpgProgrammingException(Exception):
+    """A general exception indicating a programming error."""
+
+    pass
+
+
+class GpgRuntimeError(RuntimeError):
+    """A general exception indicating a runtime error."""
+
+    pass
+
+
+class UnexpectedOutputException(GpgProgrammingException):
+    """Raised when a gpg subprocess produces unexpected output."""
+
+    pass
+
+
+class MissingLocalKeyError(GpgRuntimeError):
+    """Raised when an expected key is missing from the local keystore."""
+
+    pass
+
+
+class InvalidSignatureError(GpgRuntimeError):
+    """Raised when an invalid gpg signature is encountered."""
+
+    pass
+
+
+class VerificationFailedError(GpgRuntimeError):
+    """Raised when a gpg verfication encounters a bad signature."""
+
+    pass