v0.3.0

• feat: Retry event marshalling without contextual data if the first pass fails
• fix: Include url.Parse error in DsnParseError
• fix: Make more Scope methods safe for concurrency
• fix: Synchronize concurrent access to Hub.client
• ref: Remove mutex from Scope exported API
• ref: Remove mutex from Hub exported API
• ref: Compile regexps for filterFrames only once
• ref: Change SampleRate type to float64
• doc: Scope.Clear not safe for concurrent use
• ci: Test sentry-go with go1.13, drop go1.10

NOTE:
This version removes some of the internal APIs that landed publicly (namely Hub/Scope mutex structs) and may require (but shouldn't) some changes to your code.
It's not done through major version update, as we are still in 0.x stage.