Fix implicit PlayScript String conversions

This commit fixes several issues:
- Compiler was generating invalid IL in cases where
Convert.ImplicitPlayScriptConversion was succeeding even though
Convert.ImplicitPlayScriptConversionExists was returning false.
- Compiler was generating invalid IL b/c it was using EmptyCast instead
of BoxedCast with primitive types.
- Addressed common ambiguities introduced by allowing implicit casts
to String from any type. Handled string.Concat by always converting to
String if either side is a String, and handled Array/Vector lookups by
adding overloads for all numeric types.
Add test case for implicit String conversions
Make ExpandoObject.ToString match ActionScript behavior
Add HasOwnProperty case for IDictionary types
Fix handling of dynamic property access for Array/Vector

Updated the behavior to match ActionScript. Strings representing
integer values are converted to indexed accesses. Strings representing
any other value are dynamic property accesses for Array, and throw
ReferenceErrors for Vector.
Fix handling of in, delete, and hasOwnProperty for Array/Vector

Updated the behavior to match ActionScript. Fixes issue #276.
Merge pull request #274 from playscript/playscript-mono-bguihaire

Playscript mono bguihaire
Fix dynamic function invokes for Vector
Add test case for deleting numeric keys from Dictionary

This is to check for regressions of issue #279
Merge pull request #281 from playscript/cbroglie-strings

Fix implicit PlayScript String conversions
Remove unnecessary cast
Merge pull request #283 from playscript/cbroglie-ps-fixes

Array/Vector fixes
Fix dynamic property enumeration for perf mode Arrays
Fix Array enumerator handling when dynamic properties are null
Fix non-perf mode Array.cs compile error
Fix for issue passing null to var arg function
Make internal ActionScript classes public in PlayScript

This resolves issue #284
Re-enable some PlayScript tests which are now passing
Disable test-ps-010.play

For some reason it is passing locally but failing on the Jenkins
server due to different values for uint.MaxValue. Disabling until we
have time to sort it out.
Merge commit '99754b6350fc57d6ae89914c6ff668e8ac8323a2' into playscript-mono-bguihaire