Implement AsdfVersion class

[drdavella]

This PR directly resolves #279 by implementing a new type AsdfVersion that is used to represent all schema version information. It also implements an AsdfSpec class that will be used to represent ranges of versions, which was requested in #280.

This also directly resolves #283, in that it replaces the extern package semver with semantic_version, which is now a dependency that is expressed in setup.py. It indirectly closes #282 since we no longer want to add semver as a real dependency.

I intend to directly address #280 before closing this PR.

[edit]
Now fixes #280 too.

[drdavella]

Remember to update CHANGES.rst

[coveralls]

Coverage increased (+0.08%) to 94.385% when pulling cf116ab on drdavella:asdf-version into fd5ea50 on spacetelescope:master.

[coveralls]

Coverage increased (+0.06%) to 94.373% when pulling e6c7650 on drdavella:asdf-version into fd5ea50 on spacetelescope:master.

[drdavella]

This now resolves #280 as well (as requested by @perrygreenfield) and allows supported versions to be expressed in terms of unbounded ranges (e.g. >=X.Y.Z).

[coveralls]

Coverage increased (+0.08%) to 94.384% when pulling 1f76d9e on drdavella:asdf-version into 44b6a10 on spacetelescope:master.

[drdavella]

Since this revision removes the extern package semver, is it also okay to remove the license file licenses/LICENSE-semver.rst? It seems like the answer should be yes but I'm not really sure.

[coveralls]

Coverage increased (+0.2%) to 94.465% when pulling 7b02d52 on drdavella:asdf-version into 44b6a10 on spacetelescope:master.