Minimal support for ICU version 50

This should probably stay a branch until someone has time to generate
the minimal DLLs icudt50.dll, icuin50.dll, and icuuc50.dll.

Without these it can only be used in a project like FieldWorks
where these DLLs are already present.
Ignored test of deprecated HiraganaQuaternary no longer settable in ICU50

Also made RuleBasedCollatorTests.cs be encoded in UTF-8 so future diffs will
be meaningful.
Correct calling convention for invoking ICU50 methods

Without this debugger complains about unbalanced stack. Probably that will
cause a user-observable problem at some point.
Keep common AssemblyVersion with default branch so that Palaso won't crash
when it loads an icu.net dll built for icu50
Update AssemblyFileVersion to reflect differences.
Add an explanatory comment about why the AssemblyVersion and
AssemblyFileVersion are different.
Merge
Fix build after merge
Merge
Adding .gitattributes and .gitignore files

This is in preparation for the migration to git.