Refactorings and quality improvements

• Ensure HttpClient is threadsafe
• Code quality refactoring

Release v2.0.18

remove obsolete badge from the readme

Release v2.0.17

add some tests

Release v2.0.16

add an option to specify base url at build time

Release v2.0.15

support specifying custom client handler instance, minor code quality…

… refactorings, improve tests

Release v2.0.14

more code quality refactoring, ensure Polly message handler won't cau…

…se async deadlocks, more testing

Release v2.0.13

code quality refactorings

Release v2.0.12

Merge pull request #21 from myarichuk/develop

minor convenience methods in client builder, some more testing

Release v2.0.11

Merge pull request #19 from myarichuk/develop

add convenience methods (adding multiple certificates at once, multiple policies at once, etc)

Release v2.0.10

Merge pull request #18 from myarichuk/develop

Replace helper with BCL HashCode::Combine() function