add download function

add download function to browse page
code style fix

fix code style  and some other problem
fix confict

fix confict
TACHYON-102 Change log to use slf4j.
Change LOG.debug to use SLF4J parameterized message to reduce String concat.

Remove toString on log to use parameterized logging.

Remove toString on log to use parameterized logging.

Revert style format change in core/pom.xml.
Small format fix on pom.xml file.
add an independent servlet for downloading

To make the logic clearer, add an new servlet processing downloading
Merge pull request #431 from PasaLab/TACHYON-139

[TACHYON-139] WebUI--Add the downloading file feature on the webUI
Fix the import ordering for org.slf4j.Logger
Merge pull request #440 from hsaputra/TACHYON-102_updatelog_slf4j

Tachyon-102 Move logging to SLF4J (with log4j as backend implementation)