[BREAKING CHANGE] Field accessors should use property getters in C#

In C#, plain field accessors should not be nonparametric methods
but should be standard property getters.

The accessor methods with parameters were renamed to `GetXxx`
because a method cannot be named identically to a property.

Also, `ByteBuffer.Position`, `FlatBufferBuilder.Offset` and
`FlatBufferBuilder.DataBuffer` are now properties instead
of nonparametric accessor methods, for more idiomatic C# style.

This is a breaking change, all client C# code accessing these
fields needs to be changed (i.e. remove those `()` or add the
`Get` prefix).

Issue: #77
Change-Id: Iaabe9ada076e5ea2c69911cf6170fdda2df3487e