Support read only properties in c# 6

[arootbeer]

Following up on http://stackoverflow.com/questions/35634434/does-dapper-support-c-sharp-6-read-only-properties-in-pocos. Related to bebe80b.

[mgravell]

Beat you to it ;p bebe80b

[arootbeer]

Hey...I said bebe80b first :D

[mgravell]

1.50-beta9