Mono.Cairo: Add Context.InClip method

The cairo_in_clip function was added in cairo 1.10.
Mono.Cairo: Add Context.HasCurrentPoint property

The cairo_has_current_point function was added in cairo 1.6.
Mono.Cairo: Add Context.SetSourceColor(Color) convenience method

The Color property was obsoleted, but the suggested replacement
SetSourceRGBA has 4 double parameters, which can be unconvenient.

Add a SetSourceColor that takes a Cairo.Color parameter, make the Color
property use it, and update the message on the Obsolete attribute.
Update README.md

Add info on parallel installations which is going to be the case for 99% of people trying to compile and use mono from source.
Update README.md
Print the correct GC type if compiling using --with-gc=none.

This change released under the MIT license.
Allow building with null-gc and sgen.
Previously if --with-gc=none --with-sgen=yes was specified the sgen runtime
would include the null GC and the sgen GC which would confuse the linker.

This change released uner the MIT license.
Merge pull request #1076 from sethjackson/null-gc-msg

Print the correct GC type if compiling using --with-gc=none.
Merge pull request #1077 from sethjackson/null-gc-sgen

Allow building with null-gc and sgen.
Merge pull request #1075 from Meai1/patch-1

Update README.md
Merge pull request #1071 from bl8/cairo-additions

Cairo API additions in Context class