Implement AsdfVersion type

[drdavella]

Currently versions can be represented as either tuples or strings:

version = (1, 0, 0)
version = '1.1.0'

However, this causes issues when performing version comparisons, so generally all versions need to be converted to one format or the other. Instead, it might be helpful to have an AsdfVersion class that allows for transparent comparisons of all different version formats:

version = AsdfVersion('1.0.0')
assert version == AsdfVersion((1,0,0))
assert version == AsdfVersion(1,0,0)
assert version == '1.0.0'
assert version == (1,0,0)