Adjust scrollbar display after layout if autosized children

ScrollableControl.OnLayout sets the scroll bar display before doing the
actual layout.  If any children are autosized, they may change size as
a result of the layout, and that may change whether or not the scrollbar
is needed.  (This addresses Xamarin bug 18874.)
[llvm] Fix a few problems with longs on 32 bit platforms. Fixes #18925.
Added contribution guideline links in README 

New section has links to guideline wiki page, and the page's Important Rules section that all contributors should be aware of.
Merge pull request #987 from StephenMcConnel/bug-18874

Adjust scrollbar display after layout if autosized children
Add ICscHostObject4 skeleton
[windows] Remove sdb symbol from mono.def. Fix the build.
[VS] Change the tools version back to v110 so it works with VS 2012.
[windows] Win64 requires 16 bytes alignment too.
[runtime] Don't g_string_append an empty string when running regression tests.
[runtime] Add some low level locks to image sets to reduce contention and the usage of the loader lock.
[jit] Rewrite lookup_method () to reduce contention by not taking the loader lock while doing hashtable lookups.
[System] handle arguments whose ToString returns null in String.Format

When an argument to String.Format has no formatter, its ToString method
is called. This change takes care that the FormatHelper can handle
arguments whose ToString() method returns null.
Merge pull request #996 from DavidS/fixes/string-format-nre

[System] handle arguments whose ToString returns null in String.Format
[mcs] Primary constructors
[mcs] Initial implemntation of exception filters
Update error message
Update tests
Make all Python scripts Python 3-friendly.