Release v1.5.0 version

[sagilio]

Fix #29. Please rebase and merge.

Casbin.NET will change the version frequently than before, so I want to keep the same version between the two packages.
EFCore keeps the same version to .NET too, It is not recommended to use a different version.

Changes:

1. Namespce Casbin.NET.Adapter.EFCore => Casbin.Adapter.EFCore Casbin.NET #120,
2. Class name CasbinDbAdapter => EFCoreAdapter Casbin.NET #120,
3. Full type:

• Casbin.NET.Adapter.EFCore.CasbinRule<T> => Casbin.Adapter.EFCore.Entities.CasbinRule<T>,
• Casbin.NET.Adapter.EFCore.CasbinModelExtension => Casbin.Adapter.EFCore.Extensions.CasbinModelExtension
• Casbin.NET.Adapter.EFCore.CasbinRuleExtenstion => Casbin.Adapter.EFCore.Extensions.CasbinRuleExtenstion

4. Default table name CasbinRule => casbin_rule (CasbinRule table name - consistency across projects #29),
5. Default field name PType => ptype, V0 => v0 (Vx => vx ) (CasbinRule table name - consistency across projects #29),
6. Default add index to ptype v0 v1 v2 v3 v4 v5 fields,
7. Support .net5.0 .netstandard2.1 .netcoreapp3.1 targets,
8. Add packages icon (Casbin.NET #128).
9. Remove Travis CI support.
10. Update README for the new version.

Release Notes:

Break Changes:

1. Change namespace Casbin.NET.Adapter.EFCore to Casbin.Adapter.EFCore,
2. Change CasbinRule<T> class namespace Casbin.NET.Adapter.EFCore to Casbin.Adapter.EFCore.Entities,
3. Change class name CasbinDbAdapter to EFCoreAdapter,
4. Change default table name CasbinRule to casbin_rule, default field name PType V0 V1 V2 V3 V4 V5 to ptype, v1 v2 v3 v4 v5 .

Features:

1. Support .net5.0 .netstandard2.1 .netcoreapp3.1 targets,
2. Auto build field indexes.

Others:

1. Update Casbin.NET version v1.4.0 to 1.5.0
2. Update EFCore dependency packages version.

[hsluoyz]

@sagilio should we release v2.0.0 for these breaking changes? Actually I suggest doing the same for Casbin.NET

[sagilio]

@hsluoyz
v1.5.0 is only want to keep the same version between Casbin.NET. When multiple Casbin.NET versions are maintained at the same time later, this can better avoid version confusion.
EF Core adapter version change is v1.3.1 => v1.5.0, I think It can clearly reflect that this is a special version, and the biggest breaking change is about the table name and field name because it needs to create a new migration.

The commit of changing namespace is merged to the develop branch at Casbin.NET. Actually, it will release on the v2.0.0 version, not v1.5.0.

[hsluoyz]

lgtm