core::Engine: Fix memory leaks, misc

this PR consists of 2 parts;

1. fix compilation error while compiling using llvm(clang++ ) as reference to 'end' is ambiguous in llvm.
2. fix memory leaks in core::Engine, the following code may be useful to prove that the memory leaks do exists while reload `DataFlow` again and again:

```python
import resource
from yaafelib import Engine, FeaturePlan

def main():
    fp = FeaturePlan()
    fp.addFeature("mfcc: MFCC blockSize=1024 stepSize=1024 CepsNbCoeffs=11")
    df = fp.getDataFlow()

    for i in range(10):
        for j in range(100):
            eng = Engine()
            eng.load(df)
        print i, resource.getrusage(resource.RUSAGE_SELF).ru_maxrss

if __name__ == '__main__':
    main()

```

the output is something like:
```
0 19640
1 22808
2 25712
3 28616
4 31784
5 34688
6 37592
7 40760
8 43664
9 46568
```
if no memory leaks happened, it should be something like:
```
0 19640
1 19640
2 19640
...
```
I've check and test the code. Please review it again!

BTW, I use the great tool [valgrind](http://valgrind.org/) to find the leak code.