Allow waiting for write

Accepted pull requests from @jayvdb

• Added GitHub actions running tests (#9 )
• Allow waiting for write (#10 )

added FailingMockStream and SyncMockStream

A lot of additions (and fixes) was merged (thanks @jonhoo, @wimh, @Detegr)

• added FailingMockStream ( #2 ) and SyncMockStream (thread-safe variant of SharedMockStream - #4 )
• derived some std traits (Clone #1, Default #6,#7 )
• fixed subsequent calls to pop_bytes_written (#3)

rust 1.0.0b compatibility

Rewritten to use only stable API.
Internal implementation is changed in favor of std::io::Cursor (no public interface changes).