Subtle maintenances

Fix SMF Set ID encoding and doc updates

Safer parser and easier IE handling

CAUTION: this release contains breaking changes

Changed the value handling in QuotaValidityTime,
TimeThreshold, and SubsequentTimeThreshold to use
`time.Duration` (which were formerly `time.Time`
and `uint32`). Be sure to update them if your
code uses the constructors or value retrieval
helper functions for those IEs.

If you're not willing to use new constructors,
you can use the type-specific constructors that
are also added in this release. It  allows us to
create an arbitrary IE with given IE type and
value of the specific type. See godoc for `ie`
for what types are available and read README
(section: Creating IEs) for detailed usage.

Fix crashing issue and introduce fuzz testing

Some updates :)

Some updates in IEs

Some updates in IEs

Bugfixes etc.

Several minor updates