Fix failing unit test (conversion of strings via factors)

I was a bit surprised by this, because the bug must have been around for a while
(we need to add continuous integration for R provider and run the tests automatically!)

The problem is that the IntegerVector in recent versions of R.NET only
succeeds when the value is *not* a factor (there is an explicit check
`not (sexp.IsFactor())`) and so the pattern was never matched in the
convertor. This change uses R.NET only to check that the value is *some*
vector and then explicitly checks for numeric vectors (without ruling out
factors).
Merge pull request #89 from tpetricek/factor.fix2

Fix failing unit test (conversion of strings via factors)
Register InteropServer as a well-known singleton

Issue #86 is caused by the server object being gc'd at the server. This
can be prevented by registering the server object as a
WellKnownServiceTypeEntry and specifying that the service be provided as
a singleton. This guarantees that the lifetime of the server object will
be the lifetime of the server process.
Merge pull request #90 from dcharbon/feature/prevent-disconnection

Register InteropServer as a well-known singleton
Adopt latest R.Net for concurrency fixes

The latest R.Net 1.5.12 and RDotNet.FSharp 0.1.5 include concurrency
fixes that resolve issue #64. They might also resolve RInteropServer
disconnection issues that have been seen, though this will take some
further testing to verify.
Incorporate feedback from @tpetricek

Removed locking from RInteropServer. Removed all references to
C_StackSize since the RDotNet default is what is needed. Fixed
RWrapperGenerator.fsproj so that it doesn't use elements not supported
on Mono. Remove unnecessary reference to DesignTime assembly in Tests.