Go Agent version 0.x

Fixed premature Span.End() in apmgorm callback, causing a data-race with captured errors #229

ELASTIC_APM_SERVER_URL now defaults to "http://localhost:8200" #122
Transport.SetUserAgent method added, enabling the User-Agent to be set programatically #124
Inlined functions are now properly reported in stacktraces #127
Support for the experimental metrics API added #94
module/apmsql: SQL is parsed to generate more useful span names #129
Basic vgo module added #136
module/apmhttprouter: added a wrapper type for httprouter.Router to simplify adding routes #140
Add Transaction.Context methods for setting user IDs #144
module/apmgocql: new instrumentation module, providing an observer for gocql #148
Add ELASTIC_APM_SERVER_TIMEOUT config #157
Add ELASTIC_APM_IGNORE_URLS config #158
module/apmsql: fix a bug preventing errors from being captured #160
Introduce Tracer.StartTransactionOptions, drop variadic args from Tracer.StartTransaction #165
module/apmgorm: introduce GORM instrumentation module (#169, #170)
module/apmhttp: record outgoing request URLs in span context #172
module/apmot: introduce OpenTracing implementation #173

First release of the Go agent for Elastic APM