Update BtceApi.cs

I had issues with my nonce values getting reused when I used this library in a multithreaded application where both threads accessed the same instance of BtceApi.  To fix this I tried changing nonce++ to a thread safe increment, which required I make the nonce a long value.  After that change it seems to work better, though I haven't fully tested it.
Merge pull request #2 from patricker/patch-2

Update BtceApi.cs to add thread safe nonce increment