special case verbose=True to avoid passing invalid options to gpg --debu...

...g-level

This commit fixes #17 using one of the resolutions discussed in the issue (allows verbose=True, defaults to some valid value for gpg --debug-level, and warns user of the ambiguity and how it was resolved). This ensures that only valid values are passed to gpg --debug-level and resolves the specific error in #17.

I note that on my system's default gpg (1.4.12), calls to the gpg binary fail if --debug-level is given as an option, no matter what value (valid or otherwise) is given to --debug-level. This is unexpected, given that is is documented in the man page (although the Chanelog refers to it as a "dummy feature"). Does it behave correctly in gpg2?