Initial support for services and extensions. Incomplete, but enough to get generated unit test files to compile.

committer: Jon Skeet <skeet@pobox.com>

committer: Jon Skeet <skeet@pobox.com>

Descriptors are pretty much complete, with a slight issue of how to find the default type for a repeated field.

Patch from Miguel de Icaza: This introduces auto-detection of Mono to configure, and if found, builds and installs the runtime library, and provides a pkg-config file so developers can easily use it on Unix.

Changed fixed size methods to return unsigned integers. Finished FieldSet. Introduced mapping from FieldType to WireType and MappedType.

committer: Jon Skeet <skeet@pobox.com>