Update readme to remove notice about requiring open connections

Added Belgrade SqlClient library in the performance benchmark

In the performance benchmark is added Belgrade Sql client data access
library (https://github.com/JocaPC/CLR-Belgrade-SqlClient). This is a
wrapper around ADO.NET classes that executes queries in async manner
using callbacks. "Belgrade.Sql.Client": "0.6.2" is aded in project.json
file, and new flag "BELGRADE" controls should the test be executed.

Related: #683

TODO: All of these need race love in V2, complete re-vamp.

Add GetRowParser documentation/example

added firebird support

Add SqlMapper.RemoveTypeMap() method

Update README with documentation for [ExplicitKey] attribute

This reverts commit 91725fb3.

added firebird sql adapter to allow record insert and key retrieving

Add test DiscriminatedUnionWithMultiMapping

DiscriminatedUnionWithMultiMapping - test for check correct MultiMapping (splitOn) functionality with discriminated row parser.

Update Readme.md

Note: there's still a 1.0.0 vs 1.1.0 quirk here - digging.

Readme: Better explanation for multi mapping

GridReader.IsConsumed wouldn't be true

Non-greedy match for comments regex

Fixes #436, #280
My editor has also normalized a few line endings to LF like 99% of the
file was.

Update readme.md

Missed second bracket in ID example

Make CommandBehavior optimizations more configurable