Add Get Bytes Method Generator for C#

I updated idl_gen_general.cpp to add support for generating a Get Bytes
method for a vector to the generated C# source code. Given a byte vector
field named Foo, a method named GetFooBytes() will be generated in the
C# source code that will return an ArraySegment<byte> value referencing
the vector data in the underlying ByteBuffer.

I added a method to Table.cs named __vector_as_arraysegment that is used
by the code generated by the change to the C# generator.
__vector_as_arraysegment will take the offset of the vector and will
return the ArraySegment<byte> value corresponding to the bytes that
store the vector data.

I updated FlatBuffersExampleTests.cs to add tests to validate my
implementation of Table.__vector_as_arraysegment. I added tests to
demonstrate that the bytes for the monster's name can be extracted from
the underlying byte array. I also added tests to show that
Table.__vector_as_arraysegment returns a null value if the vector is not
present in the FlatBuffer.

I used the updated flatc.exe program to regenerate the C# source files
for the MyGame example. The new Monster class includes the GetXXXBytes
methods to return the byte arrays containing data for vectors.