Import/Export script Improvement

This is Sunny Wenhui Wu from Tsinghua University.
As a user of Apache IoTDB, I have several requests that hopes to get kindly answer from your guys.

Bug: In the 'import.bat' & 'export.bat' script, the path of MainClass is wrong, it should be changed from '.db' to '.cli'.
Requirement: The '-td' parameter of export script should be able to designate the export file name,  instead of default file name 'dump0.csv'. Currently, if the user doesn't rename the file, the new file overwrites cover the old file. Thus the user has to change name manually consistently.