Replace uses of boost::filesystem with fs

Step two in abstracting away boost::filesystem.

To repeat this, simply run:
```
git ls-files \*.cpp \*.h | xargs sed -i 's/boost::filesystem/fs/g'
```

src/bitcoind.cpp

@@ -97,7 +97,7 @@ bool AppInit(int argc, char* argv[])
 
     try
     {
-        if (!boost::filesystem::is_directory(GetDataDir(false)))
+        if (!fs::is_directory(GetDataDir(false)))
         {
             fprintf(stderr, "Error: Specified data directory \"%s\" does not exist.\n", GetArg("-datadir", "").c_str());
             return false;