Synchronize static members intialization in constructor

There are rare cases of entering an infinite loop when initializing multiple instances of TwitterImpl from multiple threads concurrently.

```implicitParamsStrMap.put(conf, implicitParamsStr);```

Might cause an infinite loop in 

```
        at java.util.HashMap.transfer(HashMap.java:484)
        at java.util.HashMap.resize(HashMap.java:463)
        at java.util.HashMap.addEntry(HashMap.java:755)
        at java.util.HashMap.put(HashMap.java:385)
```

More info about ```HashMap.put()``` causing an infinite loop can be found here http://stackoverflow.com/q/13695832/1442259