API changes and improvements

The AVP code now has its own sub-package. The old diamdict is now
diam/dict and diamtype have been moved outside the diam package.
API changes and improvements

Our sub-packages and some of their properties have been renamed to
reflect the terminology used in RFC 6733.

There are constants in the diam and avp packages that makes it easier to
write Diameter code without using numeric codes. These constants are
auto-generated using our stock base and credit-control dictionaries.

Documentation has been updated as well as travis-ci's configuration for
running the tests.

All test cases and examples still work, but programs that depend on this
package will break due to these changes and sub-package renames.
AVP is back into the main diam package

The right place for the AVP type is in the diam package. However, the
codes (auto-generated list) and flags belong to its own sub-package.

Nothing changes for AVP data formats.
Rename CMD to Command