eleminate memory leaks by the dynamically allocated na_mgr

valgrind log:

==4037== 238 (96 direct, 142 indirect) bytes in 1 blocks are definitely lost in loss record 2,403 of 2,896
==4037==    at 0x4C2B100: operator new(unsigned long) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==4037==    by 0x646A3D0: QLibraryPrivate::findOrCreate(QString const&, QString const&) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.6)
==4037==    by 0x646AB6C: QLibrary::setFileNameAndVersion(QString const&, QString const&) (in /usr/lib/x86_64-linux-gnu/libQtCore.so.4.8.6)
==4037==    by 0x60A59BE: q_resolveOpenSslSymbols() (in /usr/lib/x86_64-linux-gnu/libQtNetwork.so.4.8.6)
==4037==    by 0x609D552: QSslSocketPrivate::ensureLibraryLoaded() (in /usr/lib/x86_64-linux-gnu/libQtNetwork.so.4.8.6)
==4037==    by 0x609F918: QSslSocketPrivate::ensureInitialized() (in /usr/lib/x86_64-linux-gnu/libQtNetwork.so.4.8.6)
==4037==    by 0x60971FB: QSslConfigurationPrivate::defaultConfiguration() (in /usr/lib/x86_64-linux-gnu/libQtNetwork.so.4.8.6)
==4037==    by 0x6090AE8: QSslConfiguration::defaultConfiguration() (in /usr/lib/x86_64-linux-gnu/libQtNetwork.so.4.8.6)
==4037==    by 0x602D934: QNetworkRequest::sslConfiguration() const (in /usr/lib/x86_64-linux-gnu/libQtNetwork.so.4.8.6)
==4037==    by 0x6018332: QNetworkAccessManager::createRequest(QNetworkAccessManager::Operation, QNetworkRequest const&, QIODevice*) (in /usr/lib/x86_64-linux-gnu/libQtNetwork.so.4.8.6)
==4037==    by 0x60154E3: QNetworkAccessManager::get(QNetworkRequest const&) (in /usr/lib/x86_64-linux-gnu/libQtNetwork.so.4.8.6)
==4037==    by 0x49E813: SeafileApiClient::get(QUrl const&) (api-client.cpp:61)
==4037==
Merge pull request #363 from haiwen/eleminate_memory_leaks_2

eleminate memory leaks by the dynamically allocated na_mgr