Fastobo

Added
- `__init__` and `date` getter to `CreationDateClause` in both `fastobo.term`
and `fastobo.typedef` with proper timezone support.

Added
- Getters for `ReplacedByClause` in `fastobo.typedef`.
Fixed
- `PyFileGILRead` (used in `fastobo.iter`) should now be thread safe.

Added
- Getters for `ExpandExpressionToClause` and `ExpandAssertionToClause` in
`fastobo.typedef`.
Fixed
- Missing `NamespaceIdRuleClause` class is now a member of `fastobo.header`.

Added
- `fastobo.iter` function to iterate over the entity frames of an OBO document.
- `From<std::io::Error>` impl for `Error`.
Changed
- `OboDoc` cannot be subclassed anymore.

Added
- Getters for `IsClassLevelClause` and `IsMetadataTag` in `fastobo.typedef`.
- Getters for `RelationshipClause` in `fastobo.term`.
Changed
- Inconsistent naming for `PropertyValueClause` between `fastobo.header`
and other submodules.
Fixed
- Bug with `fastobo.header.ImportClause.reference` returning the whole clause
serialization instead.

Added
- Precompiled wheels for CPython 3.5, 3.6 and 3.7 on Windows x86-64.