Release 1.1.0

Notable changes:

• The library is now released under Apache 2.0 license
• Use Set() instead of Add() in HTTPHeadersCarrier is functionally a breaking change (fixes issue #159)
• 'golang.org/x/net/context' is replaced with 'context' from the standard library

List of all changes:

• Export StartSpanFromContextWithTracer (#214)
• Add IsGlobalTracerRegistered() to indicate if a tracer has been registered (#201)
• Use Set() instead of Add() in HTTPHeadersCarrier (#191)
• Update license to Apache 2.0 (#181)
• Replace 'golang.org/x/net/context' with 'context' (#176)
• Port of Python opentracing/harness/api_check.py to Go (#146)
• Fix race condition in MockSpan.Context() (#170)
• Add PeerHostIPv4.SetString() (#155)
• Add a Noop log field type to log to allow for optional fields (#150)